686 SUMMARY OF CURRENT RESEARCHES RELATING TO 



being overlooked entirely among the younger microscopists. As so 

 little has been written on the subject, he gives "a few simple 

 directions. 



Every objective has a certain colour with which it shows best, 

 and there is probably no object better adapted to the purpose of deter- 

 mining this colour than a well-marked Podura-soale When a 



good scale is once obtained, great care should be taken to keep it dry, 

 for when wet it is of no use. 



Now, by examining this scale with a first-class \ or higher power 

 of medium or wide aperture, it will be seen that the ' exclamation 

 marks ' are more or less coloured. Pay no attention to this at first, 

 but carefully turn the collar back and forth until the marks appear 

 sharpest and smallest. That will be the point of best correction, and 

 now the colour of the markings should be noticed. Having carefully 

 determined the exact tint of best correction, throw the objective a 

 little out of proper adjustment by turning the collar towards open 

 point or zero. This over-corrects it, and at the same time notice the 

 change in colour. The markings seem to expand, becoming hazy and 

 not at all sharp. Now turn the collar towards closed until the point 

 of best correction is passed : here the same thing is seen in regard to 

 expansion and haziness, but a different tint seems to make its appear- 

 ance. By attending very closely to this colour (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 -j- 1 ^ objective. By trial over a Po^wra-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 colour) ; by turning the collar below zero they turn greenish, 

 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 expan- 

 sion within the focal point, the objective is over-corrected, and the 

 collar should be moved towards closed." 



The author then refers to the deceptive appearances produced by 

 a want of proper correction, such as lines or network instead of dots 

 and points ; and that with homogeneous-immersion objectives without 



