190 TtlE USE OF THE MICROSCOPE 



ERRORS REGARDING OPTICAL LAWS, ETC. 



1. To use an uncorrected magnifying lens instead of a 

 corrected one. A calculation (57) shows that only a 

 corrected lens can approximate the optimum of definition 

 and illumination. 



2. To hold a corrected magnifier several inches from the 

 eye. A calculation (57) shows that this results in diminu- 

 tion of field. 



3. To put paired objectives on different instruments in 

 the Greenough binocular. As the makers (Zeiss, Spencer) 

 state, the objectives are centered to their own instruments. 



4. To use the Greenough binocular through the uneven 

 cover of a Petri dish. A disc of plate glass permits perfect 

 images. 



5. To use constantly enlargements above the maximum 

 useful magnification. They show nothing new to normally 

 acute vision. 



6. To use blue sky for high-power work. It is not intense 

 enough, and no condensers can increase the original inten- 

 sity of the source of light. 



7. Neither closing the iris of the condenser, nor moving 

 it far down should be done to alter the intensity of the light. 

 These methods decrease the aperture, and thus injure 

 resolution and definition. 



8. To use too large a source of light, especially with high 

 powers. There is glare when the light falls beyond that 

 part of the object which is seen. 



9. To omit using a yellow-green screen occasionally or 

 frequently. This cheap addition perceptibly adds to the 

 efficiency of objectives, especially of achromatic objectives. 



10. Not to use a (silvered) prism instead of a plane 

 mirror, for the best work. A plane mirror gives three or 

 more images, which are hard to focus exactly when using 

 a small diaphragm on the source, and may cause some 

 glare. 



11. To use an uncorrected condenser. Experiment 

 shows that this results in loss of aperture or in glare, usually 

 in both. 



