296 MOVEMENT 



the ground glass or along the tube of the microscope 

 according as desired. This arrangement consists of 

 a refracting prism, which is set in motion by the 

 knob P. 



On pressing this knob, the prism is brought into 

 play, and the image of the preparation is projected 

 along the tube of the microscope ; on pulling the knob 

 out, the prism is removed and the image falls directly 

 upon the ground glass or upon the sensitized plate. 



As it would be impossible to search for interesting 

 parts of the preparation from behind the apparatus 

 by watching the image upon the ground glass, this 

 portion of the operation is effected by looking through 

 the eye-piece of the obliquely placed microscope. The 

 eye-piece can be adjusted by means of a correcting 

 lens, in such a way that the image is always in focus 

 on the sensitive plate when it is in focus as viewed 

 through the eye-piece. The focussing is carried out 

 as follows, a screen of fairly thick paper is placed in 

 front of the condenser so that the light passing through 

 it does not heat the preparation, and at the same time 

 the eye applied to the microscope can easily stand the 

 illumination.* 



When the process of focussing is complete and the 

 movements are ascertained to be under favourable 

 conditions, the circular diaphragms are rotated so as 

 to obstruct the continuous light, the screen is removed, 

 the prism drawn away, and the camera set in order 

 for taking a photograph. In this case, as in the other 

 experiments described, it is as well, to take some 

 photographs on fixed plates. The moving object 

 must be strongly illuminated in front of a dark 

 background. For slight degrees of magnification 



* It would be very imprudent not to use the screen, and care should 

 l»i- taken that it is not removed during the process of focussinir; the 

 blinding light which might otherwise strike the retina would be 

 attended with grave consequences. 



