PRACTICAL PHOTO-MICROGRAPHY. 87 



fact he has of late discarded it almost entirely. Perhaps the 

 best plan is to omit the bull's-eye and use a condenser of such 

 focal length as to project on the object a sufficiently large 

 image of the radiant. 



Photo-micrography of deep objects, as many diatoms, is a 

 vexed question which the writer prefers to leave undiscussed. 

 The reader must judge for himself whether he is to get the 

 best general appearance of his object, whether he prefers to 

 resolve one plane without attention to any other plane, or 

 whether he will be best suited by a compromise, that is, by a 

 little resolution with fair general sharpness. One thing he 

 need not attempt, viz. : to get perfect resolution on several 

 planes simultaneously. 



The achievements of the most difficult photo-micrography 

 are vouchsafed only to the most careful and skillful operator. 

 There is no secret in, nor any royal road to the photography of 

 the nagellum of a microbe, or the " dots " on P. angulatum. 

 Good optical appliances, absolute freedom from tremor, 

 efficiency in centering, correcting and focusing, are the only 

 secrets. Skill in these matters can only be acquired by long, 

 earnest, unflagging study and practice. An occasional rush 

 will not do for this work ; it is necessary to give up to it the 

 entire attention for the time being, and there is no use for any 

 person to attempt this work at any odd moment, for failure is 

 sure to result. 



Above all we would counsel our reader to study the science 

 of correcting his objectives by collar or tube ; centering and 

 focusing are mechanical, and follow definite and patent rules ; 

 " correction " is never alike for two objects, and is a matter of 

 sheer accuracy of observation the highest quality a micro- 

 scopist can possess. 



ID a later chapter we shall specify certain difficult and com- 

 mon test objects giving hints and only hints how the work 

 may be attempted. 



