H2 INTERPRETATION OF APPEARANCES [ CH. Ill 



3. Use alow power of the compound microscope. 



4. Use a higher power. 



5. Use the highest power available and applicable. In this 

 way one sees the object as a whole and progressively more and more 

 details. Then as the object is viewed from two or more aspects, 

 something like a correct notion may be gained of its form and 

 structure. 



1 68. Zeiss-Greenough Binocular, Erecting Micro- 

 scope. As shown in figure 102 this consists of a microscope stage 

 with two tubes mounted side by side and moving on the same rack 

 and pinion. Either tube can be used without the other. The ocu- 

 lars are capable of greater or less separation to suit the eyes of 

 different observers. In the large cylinder near the top is placed a 

 Porro prism which erects the image. This microscope gives most 

 perfect stereoscopic images and also erect ones, and therefore is es- 

 pecially adapted for dissection and for studying objects of consider- 

 able thickness, like injected preparations etc. It is interesting to 

 note that the binocular microscope constructed by Cherubin 

 D'Orleans, 1677, was composed in like manner of two microscopes 

 side by side. It of course had no erecting prisms (For statement 

 and figure of this early binocular, see Mayall, p. 17, 18). 



169. Wenham's Binocular Microscope. This is illus- 

 trated in Figs. 103-104. There is but a single objective. The light 

 from this is divided by a prism, a part of it passing to the right and 

 a part to the left eye. That to the right eye passes directly, that to 

 the left is twice internally reflected by the prism to give it the right 

 inclination. 



In order to get the stereoscopic effect with the binocular there 

 must be an image in both eyes, and to ensure this the oculars must 

 be separable so that the eye- points are the same width as the pupils 

 of the eyes of the observer. 



One can tell whether there is binocular vision in his first trials 

 by closing first one eye and then the other. If an image is seen 

 without moving the head whichever eye is closed then of course 

 both eyes are seeing an image and one should get the appearance of 

 relief characteristic of stereoscopic images. If one does not see with 

 both eyes the eye- points are too close or too far seperated for his 

 pupils. The tubes should be seperated or approximated until each 



