THE MICROSCOPE. 



(which is the secondary spectrum), the proper correction can easily 

 be made. I can best illustrate this by the following trial: 



I have before me a ^ objective. By trial over a Podura scale 

 I find that when best adjusted the marks appear of a brilliant ruby 

 red (and most of the finest objectives which I have seen show best 

 with this color); by turning the collar towards zero they turn green- 

 ish, while, if turned towards closed, they become pink. Hence at 

 the first trial of any such object, should it appear green the collar 

 should be turned towards closed until the ruby tint appears, and if 

 too pale a red or pink, the collar should be turned towards zero. 

 By a little practice, the microscopist can tell at a glance which way 

 to turn the collar. 



There are some objects on which a correction cannot be thus 

 made; in such cases the coma must serve as a guide. The edge of 

 a red blood corpuscle will serve as a good test for practice in this 

 way. By carefully moving the collar back and forth until the edge 

 is sharp and clear, it will be seen that a brisk movement of the fine 

 adjustment causes the edge of the corpuscle to expand, both as it 

 goes beyond the focal point and also within the focal point. If the 

 correction has been made exact this expansion (coma) is equal both 

 ways, but should the greater expansion be when the object is beyond 

 the focal point the objective is under corrected and the collar should 

 be turned towards zero; but should it be the reverse, that is, the 

 greater expansion within the focal point, the objective is over cor- 

 rected and the collar should be moved towards closed. 



Very deceptive appearances are sometimes produced by a want 

 of proper correction. For example: Should an objective be just 

 capable of showing an object (such as a diatom) in dots, when cor- 

 rected; it will show it in lines if not corrected. If the dots are 

 irregularly arranged, the appearance will be that of a network, in- 

 stead of lines. This is frequently seen in any granular matter, 

 — such as the nucleus of a red blood-corpuscle of a frog — when in 

 good correction, it is shown as granular but by throwing the objec- 

 tive out of proper correction a net-work is apparently produced. 

 This should be borne well in mind when describing such objectives. 

 The same occurs with very inferior objectives even when in their 

 best correction. 



It has been claimed that homogeneous immersion objectives 

 need no adjustment. This is true only when the length of tube and 



