4t) E. M. NELSON ON BINOCULAES. 



Let US, for example, take away the compound body, eye-pieces, 

 prisms, and all other gear, leaving only the objective, which we 

 must suppose " sawn in two " (see specification of Holmes' 

 patent binocular, " R. M. S. Journal," 1870, p. 274), and apply 

 the right eye to the right half and the left eye to the left half 

 of the objective, we shall then obtain an orthostereoscopic 

 picture. This is a simple ideal case of an erect image with a 

 simple microscope ; it can be practically carried out by means 

 of Beck's prisms,* which neither " cross-over " nor transpose 

 the image. 



If we interfere in the slightest with these relations by either 

 a transposition of the image or a "cross-over," pseudostereoscopic 

 vision will be the result. While admitting: that there mis^ht be 

 a difficulty in " sawing in two" an objective and using it as 

 indicated above, we may nevertheless think of it as a practical 

 construction of simple microscope. 



I am not aware that Dr. Carpenter anywhere states the case 

 in precisely this manner, but the above is the spirit and mean- 

 ing of what he has written. 



If you take either Holmes' " sawn in two " objective, or Beck's 

 binocular simple microscope, and make an ordinary compound 

 microscope of them by appljang tubes and ordinary Huyghenian 

 eye-pieces, pseudostereoscopic vision will ensue, because you 

 will have a transposed image without a " cross-over." 



If orthostereoscopic vision is required, the transposition 

 must be corrected, and " the ordinary view " restored, either by 

 a " cross-over " or by a retransposition of the image by means 

 of transposing prisms or erecting eye-pieces. 



Now, what does Prof. Abbe say ? The cross-over is not of the 

 slightest importance ! f Yet he states it is necessary that the 

 eye-spot images (which are the images of the objective) should 

 be in a certain condition, which condition can onh^ be obtained 

 when the conditions required by Dr. Carpenter are implicitly 

 ob9|^ed. 



There is a good deal, however, that Abbe significantly leaves 



* This I have called Beck's binocular, because it is now only to be found 

 in Beck's binocular dissecting microscope ; it was, however, invented by 

 Riddell (1851), and reinvented by Wenham (1853). " R. M. S. Journal," 

 pp. 1-18 (1854). 



t " R. M. S. Journal," 1881, p. 204, " all other circumstances except this 

 one^' (the nature of the eye spots) ^^ being immaterial." 



