Chromatic Correction 75 



when re-focusing is attempted ; in a well-corrected lens all 

 of the lines will become sharp and distinct when the par- 

 ticular zone is adjusted for. 



It will be found advantageous to confirm the observa- 

 tions on the test-plate by examinations of known test 

 objects, and with practice the two together will soon enable 

 reliable estimates to be formed of the quality of objectives. 



Tests for Definition. — Use an eyepiece with the 

 objective under examination that will give a total magni- 

 fication in diameters equal to one thousand times the 

 numerical aperture of the objective — that is, if a ^-inch 

 objective, having a magnifying power of 20 diameters on 

 the 10-inch tube, had a numerical aperture of "45, an 

 eyepiece, the magnification of which was 22| diameters, 

 would be necessary to give the required 450 diameters. 

 If an objective bears this without serious breaking down 

 its definition may be considered to be good. This test 

 has the advantage of being based on a rational foundation, 

 the ratio being the same as an eyepiece power of 50 to each 

 inch of aperture in an ordinary telescope. This, again, is 

 equal to what would be seen of an object if looked at 

 through a pin-hole -gV-inch in diameter, beyond which the 

 outlines of objects fail in clearness. 



For objectives varying in power from 2 inches to ^ inch, 

 nothing is better as a test than the proboscis of a blowfly. 

 The spines in the central portion of the tongue should each 

 show a well-defined point. For high-power objectives the 

 internal markings of Triceratum and Pleurosigma angula- 

 tum, also the markings on the scales of Podura (Lepido- 

 cyrtus curvicoUis), are the most suitable. 



CHOICE OF OBJECTIVES. 



The best objectives for a novice at starting would be 

 2-inch, 1-inch, and ^-inch. The 2-inch will be found 

 extremely useful for large specimens, while the 1-inch, 

 which is considered the working-glass of the average micro- 



