of the Coal period, would, by permitting the extension of spec over wide areas and fertile soils, and by removing the press previously existing, be eminently favorable to the product of new, and especially of improved, varieties.

2. Whatever importance we may attach to the above suppo causes of change, we still require to account for the origin of specific types. This may forever elude our observation, bu: may at least hope to ascertain the external conditions favorab to their production. In order to attain even to this it will a necessary to inquire critically, with reference to every ackno ledged species, what its claims to distinctness are, so that may be enabled to distinguish specific types from mere varietie Having attained to some certainty in this, we may be prepar to inquire whether the conditions favorable to the appearan of new varieties were also those favorable to the creation of types, or the reverse-whether these conditions were those compression or expansion, or to what extent the appearance e new types may be independent of any external conditions, othe than those absolutely necessary for their existence. I am not without hope that the further study of fossil plants may enable us thus to approach to a comprehension of the laws of the crea tion, as distinguished from those of the continued existence o species.

In the present state of our knowledge we have no good ground either to limit the number of specific types beyond what a fair study of our material may warrant, or to infer that such primitive types must necessarily have been of low grade, or that progress in varietal forms has always been upward. The occurrence of such an advanced and specialized type as that of Syringoxylon, in the Middle Devonian, should guard us against these errors. The creative process may have been applicable to the highest as well as to the lowest forms, and subsequent deviations must have included degradation as well as elevation. I can conceive nothing more unreasonable than the statement sometimes made that it is illogical or even absurd to suppose that highly organized beings could have been produced except by derivation from previously existing organisms. This is begging the whole question at issue, depriving science of a noble department of inquiry on which it has as yet barely entered, and anticipating by unwarranted assertions conclusions which may perhaps suddenly dawn upon us through the inspiration of some great intellect, or may for generations to come baffle the united exertions of all the earnest promoters of natural science. Our present attitude should not be that of dogmatists, but that of patient workers content to labor for a harvest of grand generalizations which may not come till we have passed away, but which, if we are earnest and true to nature and its Creator, may reward even some of us.

CRT. LIII-On some Phenomena of Binocular Vision; by JOSEPH LECONTE, Prof. Geol. and Nat. Hist., University of California.

In giving my own explanation of the phenomena of M. Picet's experiments, I will commence with the experiment with The piece of money. If M. Pictet had made this experiment Without the median screen, it seems to me the true explanation Could not have escaped him. Let us then first try it without Che screen.

If I place a piece of money on a sheet of paper lying on the able, and look downward in the direction of the piece, but at he same time gaze on vacancy, I see two heteronymous images,

A

9.

B

8.

a a' (fig. 8), separated by an interocular space. If I now attempt to outline these images, I see also two images of the pencil. If I use the

Fright-eye image (left image) of the pencil p to draw the left-eye image (right image) of the money a', then I see one pencil tracing the outline b of the image a', while another pencil makes a tracing b' with no money in it. If I now examine the result of this experiment, I find the tracing I have made, B (fig. 9), at some distance (an interocular space) from the piece A to the right. The explanation is obvious. In gazing on vacancy, as already explained (2), the whole field of view is shifted by each eye heteronymously a half interocular space. The left-eye image of A (fig. 9) and the right-eye image of the spot B are brought together and superposed (a'b, fig. 8); while the right-eye image of A and the left-eye image of B are seen to the left and right respectively (a and b', fig. 8). It is precisely the same as the superposition of the double images of two fingers described on page 165. If, instead of using the right image of the pencil to draw the left-eye image of the money, I use corresponding images of the pencil and money, i. e., right-eye images or left-eye images of both, I find I place the pencil on the money. Finally, if I use the left-eye image p', fig. 8, of the pencil to draw the right-eye image a of the money, I find I have made a tracing an interocular distance to the left, and the result of both experiments is two tracings a double interocular distance apart.

Now in M. Pictet's mode of performing the experiment, by the use of the median screen, we cut off the right-eye image of the money a, fig. 8, and the left-eye image of the pencil p', fig. 8, and we therefore see only the left-eye image of the money a'

e

C

and the right-eye image of the pencil p; and these, being t the visual line of the two eyes, are brought together by the of corresponding points (2) precisely as the two pictures dir stereoscopic card are united, or as any two objects, an intere lar space apart, are superposed when we gaze at a distant pona If M. Pictet had used his left hand to draw, then he would hi used corresponding images of the pencil and piece; and he wou have found that in attempting to draw his illusive image would have placed his pencil on the piece.

In the 1 periment with out the media screen, fig. 1 gives the act al relation d parts, and fig 11 represents by my method the visual re sult. In the experiment with the medi an screen, the same are rep resented by figs. 12 and 13 Comparing the visual results, figs. 11 and 13, it will be observed that the additional images, viz: a and P', are cut off by the median screen.

It is evident, there fore, that in all M. Pie tet's experiments, the image we see and trace in outline is not an im age of illusion seen by the right, but a real image seen by the left, eye. The pencil we see with the right eye, and the two points, viz: the pencil and money, or the part of the paper on which we make the

drawing and the money, being in the visual lines, are brought

ogether by the law of corresponding points. In M. Pictet's xperiment these two, the pencil and the money, are similarly elated to the two eyes, one on one side and the other on the ther side of the screen-one exposed to the view of one eye nd the other to the view of the other eye. If the image we draw 3 an illusive image seen by the right eye, then the pencil with which we draw must be also an illusive image seen by the left eye. But to explain M. Pictet's experiment a little farther: When ve look directly at the money, M. Pictet says 66 we see that the verical screen is transparent throughout, and that it permits the ight eye to see the piece as through a perfectly diaphanous urface." But there are two transparent screens seen. The one een by the right eye M. Pictet observes,* the other apparently scapes his observation. The truth is, when we look at the money, he heteronymously doubled images of the median screen m m' fig. 14) meet at the distance of the point of sight. The actual relaion of parts is seen in fig. 7 (p. 322), in which A R and A Lare the isual lines converged upon the piece A. The visual result is een in fig. 14. It is seen that the visual line of the right eye tops at the right eye image of the median screen, while the left

visual line runs parallel to its image of the median screen unobstructed to the piece a'. Again, "if we give to the optic axes a direction more parallel," says M. Pictet, "we see the image of illusion move gradually to the right, traverse the screen, and come on the right side." But again, he does not observe that there are two screens seen; and again, it is the left eye image of the screen which he neglects. In truth, as the eyes become parallel, the two images of the screen, m S and m'S' fig. 14, gradually open until they become parallel, and the piece is seen between them, as already shown in fig. 13. The piece does not in the least change its position in relation to the creen seen by the left eye; only the right eye shifts its image of the creen to the left of it. If M. Pictet would place another piece f money on the right side of the screen exactly where he made he outline tracing, he would observe the two pieces unite in ne, precisely as stereoscopic pictures are united. According o M. Pictet's principles, this must be regarded as the union of wo illusive images. Where, then, are the real images?

There seems to be a kind of dexterity in the right eye. In many cases of Touble images, most persons habitually neglect the left-eye image.

It is now easily understood that by the use of lenses bef the right eye, in M. Pictet's experiments, the image is not affected not because it is illusive, but because it is the image of the i eye. But if the experiment be made without the median scree then the true right-eye image a, fig. 8, will be seen to the l and will be enlarged. The explanation of the outlining objects seen under the microscope is, of course, precisely same, as is also that of the tracing on the blank half of stereoscopic card the outline of a picture existing on the othe half.

I might illustrate the subject farther in many ways, buti seems scarcely necessary. I will only remark, in passing, the by the movement of the fields of view already explained it is easy, by voluntary squinting, to outline a piece of money in any part of a sheet of paper one may desire. I now plac a piece of money on a sheet of paper lying on the table. I place the pencil on any point where I desire to make the out line; it may be 4, 6, 10 or 12 inches from the piece. By squint ing, I now bring together the right-eye image of the pie and the left-eye image of the pencil, and then trace the outline It is a little difficult, it is true, without some small object at the point of optic convergence (point of sight) to hold the axes steady, and, therefore, to make the tracing accurate. I only speak of it to illustrate the principle of making tracings of objects at any distance from the object itself. In the case of squinting, of course a median screen is inadmissible. The phenomena of M. Pictet's first experiment, fig. 6, will

now be easily understood. If no me dian screen is used, then fig. 15 will rep resent the actual relation of parts, and fig. 16 the visual result. By compar ing these two fig ures, it will be seen that the two visual lines v v are brought together, so that the left-eye image of A. and the right-eye images of the spot b and the pencil P, fig. 15, are brought in the same line in

fig. 16; but the left-eye images of the spot b and of the pencil