34: 



l'KESIl)EXT-S ADDRESS. 



By D. W. Dennis. 



Photomicrography as it May be Practiced To-Day. 



The instnuneiit with which my \York in iiliotomifrograpliy is at pres- 

 ent being done is in a compartment of the office of Dr. C. S. Bond, of 

 Richmond, Indiana; it rests on a solid stone floor; the source of illumina- 

 tion is an arc light fed by a 52-volt alternating current. The tables, the 

 optical bench, the microscope bench and all the illuminating accessories 

 that it carries and the camera were furnished by the Bausch & Lomb 

 Optical Company; the microscope stand and all its accessories were 

 furnished b5' Zeiss; the stand is the 1899 model. The instrument is shown 

 in Fig. 1. The objectives are the 70, 35, 16, 8. 4, and 2mm; the eyepieces 

 are the 4, G, and 8 compensating and the 4 projection eyepiece. Pht 

 microscope stand is the property of the Earlham biological laboratory; 

 all other parts, including the lenses, are the property of Dr. Bond, who 

 not only by his financial assistance made it possible for me to have such 

 an apparatus with Mhich to work, but he has worked with me in all 

 that I have done, and has carried out without regard to expense every 

 suggestion that we could either of us make, with reference to the better- 

 ment of the instrument. The "we" which I use in my paper is not the 

 conventional editor's we: it means the doctor and myself. 



INTRODUCTORY. 



The photomicrography of to-day at its best has been made possible 

 by the growth of several different lines of work. The perfecting of the 

 arc light is one of these; sunlight will do instead of this, but the uncer- 

 tainty of being able to use it at any particular time is against it; the 

 arc light is always ready; its brilliancy is always the same; photo- 

 micrographs of all diameters from 4,000 down can be made with it in 

 from a very few minutes to a small fraction of one second. After one 

 has fully mastered his apparatus and needs to use the light only for 

 adjustment and exposure it is comparatively inexpensive. 



The perfecting of the microscope in all its parts was necessary before 

 the work of making photomicrographs o^'f 1.000 diameters and upwards 

 with such ease and certainty as to make them practicable for ordinary 



