STEREOSCOPIC BINOCULAR VISION 91 



forms it appears to the editor to be the wiser way to allow Dr. Car- 

 penter's treatment of the subject to stand, ami to place below it as 

 complete a digest of Professor Abbe's theory and explanation of the 

 same subject as the data before us will admit. 



The admirable invention of the stereoscope by Professor Wheat- 

 stone has led to a general appreciation of the value of the cotr/oi at 

 use of both eyes in conveying to the mind a notion of the solid form* 

 of objects, such as the use of either eye singly does not generate with 

 the like certainty or effectiveness ; and after several attempts, 

 which were attended with various degrees of success, the principle of 

 the stereoscope has now been applied to the microscope, with an 

 advantage which those only can truly estimate who (like the Author) 

 have been for some time accustomed to work with the stereoscopic 

 binocular 1 upon objects that are peculiarly adapted to its powers. 

 As the result of this application cannot be rightly understood with- 

 out some knowledge of one of the fundamental principles of binocular 

 vision, a brief account of this will be -here introduced. All vision 

 depends in the first instance on the formation of a picture of the 

 object upon the retina of the eye, just as the camera obscura forms 

 a picture upon the ground glass placed in the focus of its lens. But 

 the two images that are formed by the two eyes respectively of any 

 solid object that is placed at no great distance in front of them are 

 far from being identical, the perspective projection of the object 

 varying with the point of view from which it is seen. Of this the 

 reader may easily convince himself by holding up a thin book in 

 such a position that its back shall be at a moderate distance in front 

 of the nose, and by looking at the book, first with one eye and then 

 with the other ; for he will find that the two views he thus obtains 

 are essentially different, so that if he were to represent the book as 

 he actually sees it with each eye, the two pictures would by no 

 means correspond. Yet on looking at the object with the two eyes 

 conjointly, there is no confusion between the images, nor does the 

 mind dwell on either of them singly ; but froin the blending of the 

 two a conception is gained of a solid projecting body, such as could 

 only be otherwise acquired by the sense of touch. Now if, instead 

 of looking at the solid object itself, we look with the right and left 

 eyes respectively at pictures of the object, corresponding to those 

 which would be formed by it on the retina? of the tw r o eyes if it were 

 placed at a moderate distance in front of them, and these visual 

 pictures are brought into coincidence, the same conception of a solid 

 projecting form is generated in the mind, as if the object itself were 

 there. The stereoscope whether in the forms originally devised by 

 Professor Wheatstone or in the popular modification long subse 

 quently introduced by Sir D. Brewster simply serves to bring to 

 the two eyes, either by reflexion from mirrors or by refraction 

 through prisms or lenses, 'the two dissimilar pictures which would 

 accurately represent the solid object as seen by the two eyes respec- 



1 It has become necessary to distinguish the binocular microscope which gives 

 true stereoscopic effects by the combination of two dissimilar pictures from a 

 binocular which simply enables us to look with both eyes at images which are 

 essentially identical (p. 106). 



