ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 627 



moderate in first cost and inexpensive in use from the size of the 

 plate used. Though the plate is small, 3^ X 4 in. (lantern size), it 

 is very useful and will meet most of the needs of the amateur workers 

 for whose convenience the instrument is intended. 



There is a class of work of which this little camera is incapable, 

 and in introducing it to the notice of microscopists, it is not intended 

 to convey the impression that it will supersede other means where 

 skilled hands and elaborate apparatus are absolutely necessary. To 

 those who have but an hour or two of an evening for observation with 

 the Microscope, this camera may prove of service in securing a photo- 

 graph quickly at the work-table. 



The box above the cone might be dispensed with, and the slide 

 carrying the ground glass attached directly to the large end of the 

 cone. The advantage in having the box is the shutter, which may be 

 fitted to its interior for excluding light from the plate at the moment 

 of completing the exposure, a preferable means to that of placing a 

 piece of black paper between the objective and the source of light. 

 Instead of having the ground glass and plate-carrier in one frame, it 

 might be desirable for some to have them separate, having more than 

 one plate-holder. The apparatus can at a trifling cost be attached to 

 most stands, and when properly made should not exceed, including 

 ground glass and plate-holder, seven or eight ounces in weight." 



Photographing Bacillus tuberculosis.* — M. Defrenne describes 

 the process which he adopts to photograph this Bacillus with a Tolles' 

 1/10 in, (hom. imm.), without eye-piece, using extra rapid bromo- 

 gelatine plates, developed with ferro-oxalate, a petroleum lamp being 

 employed for illumination. 



If, he says, the determination of the actinic focus of objectives 

 constitutes, so to say, the chief difficulty in photographing ordinary 

 microscopic preparations, it is no longer so when we deal with 

 organisms so infinitesimally small as the bacilli of tuberculosis. 

 Here arises a difficulty of quite another kind, which at first seemed 

 insurmountable : the staining of the bacilli by means of fuchsin. 

 This agent, even when it is employed in thick layers, is somewhat 

 actinic, and it becomes the more so as the object stained is smaller or 

 more transparent. These two circumstances are combined in the highest 

 degree in the organisms in question. Thus at the beginning the 

 plates exposed were either uniformly acted on or the image was so 

 faint and so little difi'erentiated after development that they were 

 worthless for proofs on glass or on paper. 



These negative results suggested the abandonment of the attempt, 

 when the idea was suggested of having recourse to the use of a com- 

 pemaiing glass of a colour complementary to red (that is green), placed 

 between the objective and the sensitized plate. By thus filtering the 

 imago formed by the objective, the red rays, the only ones passing 

 through the bacilli, are absorbed, if not wholly, at least in great part. 

 The microbes therefore appear nearly black on the plate, and make 



♦ Bull. Soc. Belg. Micr., x. (1884) pp. 128-35:!. 



