72 THE USE OF THE MICROSCOPE 



This method has the folio whig disadvantages: (1) it 

 requires, for accurate work, a specially calculated bull's-eye 

 condenser; (2) combinations of lenses balsamed together 

 cannot well be used on account of the heat; (3) the small 

 diaphragms cannot always be got near enough to the 

 source of hght; (4) on changing the electric bulb, a process 

 of accurate centering and distancing of the source must be 

 gone through; (5) the bull's eye must be fixed to the 

 lamp, and both microscope and lamp fixed to a board, 

 so as to keep the centering exact; (6) arrangements must 

 be made so that the distance of the condensing lens from 

 the microscope mirror does not change when the microscope 

 is inclined. 



Errors in lamp distance or bull's-eye distance will, 

 in the writer's experience, lower the aperture of the con- 

 denser cone. The corrections and adjustment of the 

 condenser have to be fitted to a fixed distance of the bull's 

 eye from the source. 



In the second method of using a bull's eye, the corrected 

 lens is put at its focal length from the source, and the 

 condenser focused for parallel rays. An iris is near the 

 surface of the bull's eye. The lamp must be far enough 

 away for the edges of the diaphragm to be fairly sharp. 

 A good photographic objective, with its outer surface next 

 the microscope, is the best bull's eye (Hartridge). This 

 method also requires perfect and permanent centering 

 of lamp, bull's eye, and microscope, which must be fixed 

 to a board. There is a difficulty in focusing the condenser 

 for parallel rays, since it has no diaphragm to focus on. 

 This method has fewer disadvantages than the first way 

 of using the bull's eye, but still requires too many 

 adjustments. 



A third method of using a bull's eye, in which an image 

 of the source is formed in the condenser iris (or properly 

 in the front focal plane, which is usually within the con- 

 denser), cannot, in the writer's opinion, be used advan- 

 tageously with an ordinary corrected condenser; l:)ecause 

 the light passes through in a way for which no correction 



