626 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



As shown in fig. 103, the apparatus consists of a comparatively small 

 but fully corrected condensing lens C C, mounted in close proximity to 

 an iris diaphragm I L About 12 in. from the condensing lens, there is 

 mounted an auxihary iris A A, and close to this a simple lens L L. The 

 Microscope is set up at such a distance from the auxiliary iris that its 

 substage condenser S is in the right position to throw a sharp image of 

 the auxiliary iris upon the object to be photographed. The adjustment 

 of the apparatus is such that the main condensing lens C C throws a 

 sharp image of the source of light centrally upon the auxiliary iris A A, 

 which latter image is then focused by the substage condenser S upon 

 the object on the stage. The auxiliary simple lens L L is approximately 

 of the right power to form an image of the iris 1 1 upon the back lens 

 B B of the substage condenser. The purpose of this lens is thus to 

 prevent the spreading out of the light passing from the source through 

 the condenser C C by bending the cones of rays near the auxiliary iris 

 A A, so as to direct them centrally upon the substage condenser in the 

 manner clearly shown in the diagram. The characteristic feature is, 

 therefore, that all scattering of the light is completely prevented. 



Fic4. 103. 



To adjust the apparatus, the auxiliary iris diaphragm should first be 

 placed at a distance from the substage condenser equal to the distance 

 from the lamp flame for which the substage condenser is corrected — 

 generally about 8 inches. 



The substage condenser should next be centred, after which the 

 auxiliary iris should be closed and its image focused centrally over the 

 object to be photographed by using the centring screws of the auxiliary 

 irisj and the focusing movement of the substage condenser. The next 

 step is to adjust and focus the source of light and the Watson-Conrady 

 condenser, so as to produce an enlarged image of the brightest part of 

 the source of light centrally on the auxiliary iris. The iris of the 

 Watson-Conrady condenser is now to be closed so as to fill the substage 

 condenser completely with light whilst preventing an excess of light from 

 flooding the surroundings. A satisfactory image may then be produced 

 on the focusing screen in the usual way, but before making an exposure 

 the auxiliary iris should be carefully closed until it begins to reduce the 

 size of the picture on the screen. This final step shuts off all false light 

 from the outer parts of the object, beyond the portion to be photo- 

 graphed. 



