ILLUMINATION 07 



corrected bull's eye and focusing them with the condenser; 

 and, lastly (5), focusing by a corrected bull's eye the source 

 of light in the iris of the condenser, and then focusing 

 the bull's eye on the slide by the condenser. The first of 

 these methods has long been expounded by Nelson; the 

 second is preferred in practice by Hartridge and the 

 writer (Fig. 16); the third and fourth were more or less 

 favored by Spitta and Coles; and the fifth has been advo- 

 cated by Koehler, especially for photography. 



Use of Original Source of Light. — No doubt, the original 

 incandescent source of Ught, properly diaphragmed to a 

 circle of the right size, and focused by a corrected condenser 

 on the object, gives an unexcelled illumination, with 

 maximum intensity and aperture. For ordinary work, 

 the image of this circle on the slide should coincide with 

 the object-field. Under these circumstances, the objective 

 aperture circle at the back of the objective (in the absence 

 of strong and wide diffraction from the object) is black, 

 outside of the illuminated aperture circle of the condenser. 

 By shifting a series of diaphragms, in blackened card or 

 thin metal, before the source, this adjustment for size 

 of source of light can be made whenever an objective or 

 eyepiece is changed. It is an important adjustment, and 

 adds noticeably to the clearness of the image, allowing 

 a higher aperture to be used in the condenser. The 

 writer can do his high-power work with useful magnifi- 

 cations from 200 to 1,350 with a maximum diameter of 

 22 millimeters for the source of light, at 25 centimeters 

 distance. Possibly, a kerosene or acetylene flame could be 

 found in which the luminous part allowed of a diaphragm 

 of this diameter (or less, if the lamp is closer). If so, it 

 would seem to be an unexcelled source of Ught. Or the 

 direct flame could be used only for the high powers, and a 

 ground glass turned in for the low powers. 



For optimum vision, as Nelson (101) partly foresaw, 

 and as Beck (32) and Hartridge (71) have shown, the image 

 of the source, as projected on the slide, must be smaller 

 than the object-field. Perhaps less than one-fifth the 



