230 SUMMARY OF CURRENT RESEARCHES RELATING TO 



depends on the free aperture of the objective, that when part of an 

 objective only is utilised, the shape of the diffusion disks is the same as 

 that of the part used (which had not hitherto been adequately recog- 

 nised), and that their position is likewise determined by the part of the 

 objective used, e.g. when the right half only of an objective is used, 

 points nearer than the plane in true focus get shifted to the right, points 

 further away get shifted to the left. Then follows a lengthy review 

 and comparison of the various causes which operate to give the im- 

 pression of solidity and plasticity in naked- eye vision, and those which 

 come into play in stereoscopic vision with the Microscope. The subject 

 is treated very fully, experiments being suggested to illustrate the various 

 points. After this comes a discussion of the various forms of binocular 

 Microscopes. They are divided into two classes, those in which separate 

 objectives are used — of which the Greenough Microscope, made by 

 Messrs. Zeiss, is the best example — and those in which separate parts 

 of a single objective are utilised, as in the Riddell, Wenham, and 

 Stephenson form of binocular, and the Al)be stereoscopic eye-piece. 

 Regarding the second class, it is pointed out that : 



" On this very simple property, that parts of an objective used by 

 themselves bodily shift the image of any area lying in a plane at right 

 angles to the optic axis of the whole lens, without anij change of actual 

 shape, depends the stereoscopic effect of the contrivances we are con- 

 sidering. If we may talk of the different parts of an objective as 

 ' looking at ' an object, we might say that no separate part of an objective 

 can ' look ' along any other direction than one parallel to the optic axis 

 of the whole lens — a very different matter from ' looking at ' the object 

 from the actual direction of the part of the object utilised, in which 

 case the object squares we are considering just now would be fore- 

 shortened, and assume different shapes according to the point from 

 which they were regarded. Helmholtz appears to have recognised, 

 almost half a century ago, the peculiar manner in which the different 

 parts of an objective ' look at ' and ' see ' the object, for in his ' Physio- 

 logical Optics ' the action of Nachet's binocular Microscope is explained 

 as due to the causes stated, in a few crisp and short sentences. But no 

 better proof can be given that his explanations were not understood 

 till a much later date, than that NaegeK and Schwendener, in their well- 

 known work on the Microscope, dismiss Helmholtz' remarks in a short 

 footnote as being incorrect. 



" The first to explain the whole matter at length was Abbe, who, in 

 a series of papers in 1881 and 1882, notably in his paper ' On the Mode 

 of Vision with Objectives of Wide Aperture,' clearly showed how the 

 lateral shifting of the images of different planes of the object by different 

 parts of the objective constitutes a particular form of parallactic displace- 

 ments." 



Reference is then made to the controversies on the subject both 

 before and after Abbe's paper, the latter, which the author follows, 

 being explained in detail, and it is shown that the truth of the theory is 

 amply confirmed by the diversified action which can be obtained by the 

 Abbe binocular eyepiece. 



Being led to a study of the subject of stereoscopic effect with the 



