35 



purposes was possible. Indeed, the proper focusing of the microscope 

 has been made so easy by Zeiss's latest stand that it may be said that 

 only within a few months past has the use of these high powers been 

 available except in the hands of the foremost experts, and even these 

 consumed so much time and made so many failures to every success 

 that a good photomicrograph was as costly as it was rare; an entire 

 revolution of the micrometer adjustment screw in Zeiss's new 1899 model 

 stand for photomicrography lifts or lowers the tube only .04 of a milli- 

 meter, i. e., one-fiftieth of the entire focal distance, and since a movement 

 through less than one degree is entirely practicable, the tube of the 

 microscope can be raised or lowered one nine-thousandth of a millimeter, 

 or one two hundred and twenty-five thousandth of an inch. This is one 

 eighteen thousandth of its focal distance. 



How correctly to illuminate the object is again a science in itself; 

 unless this is done, the most complete and costly apparatus constructible or 

 imaginable will not give one correct photomicrograph; if the illumination 

 is nearly right the results will be entirely wrong; the object can 

 be drowned in light or it can be surrounded with halos that will remind 

 the operator of a medieval painting without a suggestion of the piety 

 that should accompany the reminder. 



The production of a good photomicrograph requires a woriiing knowl- 

 edge of photography; the use of the riglit developer, the right plate, the 

 proper use of reduction and intensification of the negative— all affect 

 details. Three or at least two experts have hitherto been necessary for 

 the production of a good photomicrograph of 2,000 or more diameters— a 

 physicist to illuminate it, a microscopist with a knowledge of the object 

 to adjust and focus the microscope, and a photographer to expose, develop 

 and print it. The inti'oductions to all atlases of this sort that I have seen 

 show that the skill of several men has been enlisted in their production. 



Photomicrography has grown then with the growth of microscopy, 

 photography, and optics; it has proposed problems to all these sciences 

 which they have separately taken up and solved in its behalf. 



To retrace the steps from Daguerre to the end of the century, from 

 Newton to Abjje, from the Dutch spectacle maker to Zeiss, is the work of 

 books, not addresses; the sacrifices and victories along these journeys may 

 have been elsewhere equaled, they have not been surpassed. 



