26 Modern Microscopy 



actuates a fine screw which causes a movement of the 

 body -g^^ inch for each complete turn. At any moment 

 either milled head may be used and time thereby saved 

 when low powers are employed. 



Where a simpler method may be desired, practically the 

 same result can be secured by having attached to the centre 

 of the ordinary milled-head a smaller spindle, say, one-sixth 

 of the diameter of the former. When a quick movement 

 is required this spindle can be turned between the fingers, 

 and a rate equal to six to one of that obtainable by slowly 

 rotating the ordinary milled-head will be secured. Messrs. 

 Watson and Sons have applied this to their instruments, and 

 the writer has found the working exceedingly satisfactory. 

 When testing the performance of the fine adjustment, a 

 central cone of light must be used ; if the light be thrown 

 obliquely there will be of necessity an apparent movement 

 in the direction from which the light comes. With central 

 illumination there should be no shake or displacement 

 whatever in the object when it is focused. 



Nearly every maker has his own system or systems of 

 fine adjustment, possessing features more 

 or less desirable, but they are mostly 

 modifications of those mentioned here. 

 Some firms adopt a most excellent form 

 of fine adjustment for a superior class of 

 microscope, while in the students' patterns 

 the method employed is dissimilar and 

 oftentimes useless for high-class work. It 



Fig 7 — Spindle ^°'^^*^ ^^ ^^^ better that efficiency were not 



Milled Head sacrificed in such a manner for the small 



Seeds'tTfi^e «^ving in cost involved. 



Adjustment. Above all things avoid the form of fine 



adjustment which carries the whole weight 



of the body of the instrument, or depresses it against a 



spring, as in the Continental instruments and cheap 



students' forms : these are almost worse than no fine 



adjustment at all, as they invariably soon work loose in the 



fittings and cause great annoyance. 



