220 PRACTICAL MICROSCOPY. 



as when using the sun's rays as before described. The 

 same focussing apparatus is employed, and the ammonio- 

 sulphate cell should invariably be inserted, but the ground 

 glass is never necessary. To produce negatives of tissue- 

 preparations with 500 diameters it requires exposures of 

 about three minutes. Other powers require proportionate 

 exposures. 



Magnesium wire costs 12s. 6d. per ounce; two ounces 

 will, with care, answer for three or four hours' constant 

 work, and ought to produce from twelve to thirty nega- 

 tives, in accordance with the difficulties of the subjects to 

 be represented. The fumes of magnesia resulting from the 

 combustion are carried into a chimney five feet long, made 

 of a spiral wire covered with muslin, terminating in a muslin 

 bag, shown in the figure, in which the oxide condenses, while 

 the draught goes on through the interstices of the muslin. 



The method of photographing such objects as Amphi- 

 pleura pellucida and Nobert's test-lines has also been 

 described by Dr. Woodward in a letter to M. Deby, a 

 vice-president of the Belgian Microscopical Society. Such 

 objects as the A. pellucida require the use of very oblique 

 light, which is obtained in the following manner : A parallel 

 pencil of solar rays, from the heliostat and plane mirror, is 

 intercepted by a blue cell and diaphragm, which only 

 allows a circular pencil of half an inch diameter to pass. 

 The light enters parallel to the optic axis of the micro- 

 scope, placed in the usual position for photography, but at 

 the lateral distance to the right or left of 3 inches. If 

 the light is intercepted by a large achromatic prism, of 

 a focal length of about 3 inches, the desired obliquity 

 can be obtained without difficulty. The best result is 

 obtained when the rays are concentrated to a focus upon 

 the object, and it is indispensable that the stage of the 

 microscope be as thin as possible. The illumination thus 



