224 THEORY OF MICROSCOPIC OBSERVATION. 



depressions as such, for instance, the binocular stereoscopic 

 Microscopes of Nachet, but those binoculars in which the images 

 are seen in reversed order (B A instead of A B) will produce the 

 opposite impression exhibiting therefore the elevations as depres- 

 sions, and conversely. This pseudoscopic appearance was indeed, as 

 Harting reminds us, observed in 1853 both by Riddell in his catoptric, 

 as well as by Wenham in his dioptric binocular Microscope. 1 The 

 conditions which produce the pseudoscopic effect, as well as the 

 means for its elimination, have now become generally known ; 

 many objects are, however, wholly unadapted for binocular observa- 

 tions, since they produce under all circumstances bizarre images. 



Among the contrivances by which the true stereoscopic image can 

 be changed to a pseudoscopic one as desired, that devised by Nachet 

 is simple and convenient. Nachet has, for 

 several years, made stereoscopic Microscopes 

 with a movable prism, mounted so that the 

 changing of the images is effected by a sliding 

 motion. The prism is constructed as shown in 

 / Fig. 123, and is placed over the objective, so 



that the horizontal projection of the oblique 

 surface P (45 to the horizon) covers half the 

 f c ^m>) aperture. According as the oblique face of the 



prism is adjusted to reflect one or the other 

 half of the image pencils, the same eye will see 

 FIG. 123. th e image formed by the right or the left half 



of the objective. By the movement of the 

 prism the stereoscopic effect is altered (supposing the object is 

 adapted for observations of this kind), and what in the one 

 case appeared as an elevation, is represented in the other as a 

 depression. 2 This arrangement is specially recommended for the 

 production of stereoscopic photomicrographs, because it is possible 

 to employ one and the same tube, without alteration of adjustment, 

 for the reception of the two images. 



1 Harting: "Das Mikroskop," 1st ed. pp. 195 and 775; 2nd ed. i. p. 199, 

 iii. p. 240. 



2 Numerous examples of this are supplied by Valentin: " Archiv fur 

 mikr. Anat." vi. (1870) p. 581. They belong, however, rather to the theory 

 of the stereoscope than to the theory of microscopic observation. The "great 

 future" which Valentin prophesies for the binocular Microscope, especially 

 for the more accurate examination of crystals, we have not at present much 

 faith in. 



