Ti^.70 



50 E. M. NELSON ON BINOCULARS. 



the eye spots is |/\|q ora, |/\|0 oin, and as the images are precisely 

 similar the binocular is non-stereoscopic. By means of semi- 

 circular eye-caps the inner halves of the eye-spot images are 

 cut out, a differential image is thus obtained, and a " cross-over " 

 effected, which results in orthostereoscopic vision, because the 

 transposition is corrected. If the outer halves are cut out there 

 is no " cross-over," and as the transposition remains, pseudo- 

 stereoscopic vision is the result. 



Dr. Mercer* has pointed ont that the inner halves of the 

 eye- spots may be cut out by screw- 

 ing in the tubes (Fig. 10) and the 

 outer cut out by screwing them 

 apart. This method of using the Abbe eye-piece is preferred 

 by some to the employment of the semi-circular caps, which 

 they find uncomfortable. 



In Beck's binocular simple microscope " the ordinary view " 

 is not interfered with, and consequently it is orthostereoscopic ; 

 the objective is divided by prisms. Cherubin d'Orleans (1677), 

 an early binocular, is pseudostereoscopic because there is no 

 " cross-over," and the microscope transposes and inverts the 

 image ; the letters l and ii denote the left and right hand 

 objectives. 



Holmes' " sawn in two " objective was, as first made, pseudo- 

 stereoscopic, the image being transposed and inverted without 

 a " cross-over ; " afterwards it was fitted with erecting eye- 

 pieces, which restored "the ordinary view" and gave an 

 orthostereoscopic image. , 



I^achet I.f The first is a three-prism form with inversion, 

 transposition, and a " cross-over." 



Nachet II. is a two-prism form, which transposes and inverts; 

 it is constructed so that by a movement of the prism it will 

 yield either a "cross-over" or no "cross-over, "and consequently 

 both " ortho- " and "pseudo-" stereoscopic images can be 

 obtained. In this as well as in the preceding form a prism 

 divides the objective. 



Powell is, strictly speaking, a non -stereoscopic binocular, 

 the images in each tube being precisely similar. It was 



* "Proceed. Amer. Soc. Microscopists," 1882, p. 129, and " E. M. S. 

 Journal," 1882, p. 271. 



t " Journal K. M. S.," 18,54, p. 74. 



