296 The Mac/nesium and Electric Liglit [IJ^IJlIlliVjETJTS' 



When the focussing is completed, the rod is removed. I have fomid 

 this simple and cheap arrangement superior in delicacy and convenience 

 to any of the more costly arrangements I have heretofore tried. 



The chemical processes employed in taking the negatives do 

 not diiier in any respect from those used in ordinary photographic 

 work, and I have found that by employing a practical photographer, 

 allowing him to manage the dark room, and confining my whole 

 attention to the optical arrangements, I not only get many times 

 more pictures in a day, but they are much better than can be pro- 

 duced by anyone who attempts to do the photographic work, as well 

 as manage the microscope himself. 



I find myself thus enabled to sit down quietly of an evening, 

 and dui'ing four houi-s' work to produce fi'om twelve to thirty nega- 

 tive ? or more, in accordance with the difiiculty of the subjects and 

 my pre^dous knowledge of them. Any microscopist who is willing 

 to go to the moderate expense of battery and lamp, and to add two 

 or three specially-constructed objectives to his microscopical appa- 

 ratus, can, by employing a photographer one or two evenings in 

 the month, reproduce all the more interesting of his month's ob- 

 servations with a decree of economy and beauty not to be obtained 

 by any other means; and if he follows the method I have above 

 described, the character of his results will be conditioned by his 

 skill as a microscopist rather than by any other circumstance. As 

 to the time of exposure requu-ed for taking negatives with the 

 Electric hght, I find that for 1000 diameters about thirty seconds 

 is necessary for that class of objects (such as Angulatum, the 

 Nobert's plate, &c.) for which it is not necessary to employ a 

 groimd-glass plate to prevent interference phenomena. In photo- 

 graphing the soft tissues and many other objects, it is necessary to 

 insert a piece of ground glass below the achi'omatic condenser, to 

 escape the interference phenomena which else occur, precisely as 

 must be done in photographing the same objects by sunlight. This 

 increases the time of exposure to about three minutes for 1000 

 diameters. Other powers require proportional times. 



2. The Magnesium hght affords a beautiful source of illumina- 

 tion comparable to white-cloud illumination of the best character, 

 or to the light of the sun after it has passed through a sheet of 

 groimd glass. Without the use of ground glass, this light serves 

 admirably for the production of photographs of the soft tissues with 

 any power under 1000 diameters. The hght being composed of 

 a mixed pencil, with rays passing in all directions, there are no 

 interference phenomena; but for the same reason, on the Robert's 

 plate and many test-objects, the results are inferior to those pro- 

 duced by the sim or by the Electric light; with powers much 

 liigher than 1000 diameters, however, the time of exposure becomes 

 inconveniently long. 



