184 HOW TO SEE WITH THE MICROSCOPE. 



effects without than with it. The former condensers 

 Avere generally of short focal length, and of consider- 

 able aperture. In the late ^tands having swing sub- 

 stages, it being possible to swing sub-stage and con- 

 denser together bodily, there seems to be no further use 

 for condensers of wide angles, while on the other hand 

 one is enabled to use in the place thereof cheaper and 

 much less expensive instruments, and the lower the 

 angle the better, and one need not be very particular 

 as to the matter of achromatism. 



On commencing the use of the little Histological, it 

 occurred to the author (and probably to scores of others) 

 that its swinging stage was a strong invitation to experi- 

 ment again with sub-stage condensers, not for the pur- 

 pose of resolving difficult tests by extremely oblique 

 illumination, for in this work the achromatic condenser 

 is of no manner of account, but, per contra, it seemed 

 obvious that by the use of a narrow apertured lens 

 placed below the stage, and so arranged that its inclina- 

 tion might be changed at will, good effects might be 

 secured in two directions : First, by the concentration 

 of a narrow cone of light immediately upon the partic- 

 ular portion of the object under examination, thus en- 

 abling the observer to sharply illuminate a certain point 

 of his object, and with less danger of drowning out 

 details in a general flood of light. Secondly, such a 

 contrivance would do good service by daylight in dark 

 and rainy weather. It required but few experiments to 

 demonstrate that there was force in the above reasoning, 

 and the next thing in course was to ascertain what par- 



