NATURAL PHILOSOPHY. 151 



(see Annual Scientific Discovery, ISoS), has not worked as well as was 

 expected, inasmuch as the prepared plates were found to require too much 

 assistance from the engraver's scraper in order to perfect them. A company 

 formed in England to carry out the process upon a large scale, has proved, 

 commercially, a failure. Several processes of photographic engraving are, 

 however, now used to some extent in Europe; the best of which is that of 

 Xie'pce St. Victor, of Paris, whose results resemble an aquatint engraving, 

 and are very beautiful. The great drawback encountered by all who have 

 worked in this field thus far, appears to be, the small number of perfect 

 impressions which the photographically engraved plates are capable of 

 printing. 



BINOCULAR VISION. 



Of the thousands who gaze with delight upon the magical effects produced 

 by that small instrument known as the stereoscope, how few there are who 

 comprehend, or attempt to assign reasons for, the extraordinary optical illu- 

 sions experienced through its instrumentality! 



It is with the view of, in some degree, elucidating the principles of vision, 

 upon which these are founded, that the following article is written. 



It will, in the first place, be well to consider the difference between monoc- 

 ular and binocular vision. Nature has furnished us with several means of 

 determining the distance of objects which may happen to come within reach 

 of our visual organs. One is, that of distinctness ; a greater or less degree 

 of which other things being equal gives an idea of greater or lesser 

 distance in the object viewed. The second is through the change of focus 

 required in the lens of the eye in refracting to a point on the retina, rays of 

 light entering it with a greater or lesser degree of parallelism, thus producing 

 in the brain a consciousness of unequal distances in the objects from which 

 they emanate. 



The means above alluded to, it is evident, are enjoyed in almost the same 

 degree when viewing with one eye as where both are used. By far, however, 

 the greatest pOAver with which nature has endowed us of discriminating 

 distances, is through the agency of binocular vision; or, in other words, in 

 the sensation produced in the brain by the different degrees of convergcncy 

 of the optic axes required in obtaining distinct vision of the differently dis- 

 tant points of objects upon which they are directed. It is to this faculty 

 that we are indebted for our most palpable evidence of differential distances, 

 and for that consciousness of solidity and relief so remarkably experienced 

 in the stereoscope. It is evident, for example, when we are looking at a 

 house or other object that has depth as well as breadth, from such a point of 

 view as to enable us to see two sides of it at once, that we receive a differ- 

 ently perspective image upon the retina of either eye, or that we must see 

 more of one side and less of the other with the right eye than with the left, 

 or vice versa, thus accomplishing with one view what a person with but 

 one eye would require two views at positions 2 1-2 inches apart the distance 

 between the eyes to accomplish. These are the differently perspective 

 views of the stereoscopic cards, and it is the effort to reconcile these dissim- 

 ilar pictures by converging the optic axes at points differently distant from 

 the eyes which produces the wonderful effect above alluded to, and which 

 enables us to experience all the sensations of delight which would be pro- 

 duced by the contemplation of the landscape itself. The stereoscopic pic- 

 tures will, of course, never quite correspond. They are taken simultaneously 



