1900J MICROSCOPICAL JOURNAL. 193 



Illumination. — Sunlight is par excellence the best 

 source of illumination for phoomicrography. A good 

 substitute for a heliostadt is a fair-sized mirror swinging 

 on a double axis, and capable of being regulated by hand. 

 No difficulty is experienced in keeping the light centred, 

 as exposures by sunlight, are of such short duration. 

 When using sunlight, care should be taken to pass the 

 rays through a cell of saturated solution of alum, in order 

 to absorb the heat rays, otherwise serious damage may be 

 done to the objective and the sub-stage condenser. After 

 sun-light, diffused daylight from a window with a north- 

 ern exposure is the next best light at the disposal of the 

 photo-microscopist, but when it is necessary to use artifi- 

 cial illumination, acetylene gas or magnesium wire will be 

 found to give satisfactory results. Some objects are bet- 

 ter shown under a diffused light, such as may be obtain- 

 ed by the interposition of a ground-glass screen, or near 

 a window without the aid of a condenser. If the color of 

 the object be dark, or reflects but little light, the bull's- 

 eye should be focussed on the specimen, care being taken 

 to avoid glare or excess of illumination, which will result 

 in a confused image in the negative. With some objects, 

 Lieberkuhn may be used advantageously, with others the 

 parabolic reflector, but the majority yield better results 

 under the most simple forms of illumination. 



Media. — Potato-agar is suggested as a good cultivating 

 medium for thermophilous bacteria. It is prepared as fol- 

 lows : — Potatoes are steamed, pealed, and pounded. To 

 100 grammes of potato add one litre of water, steam the 

 mass for half an hour and then filter. To the filtrate add 

 two per cent of agar and autoclave the whole for fifteen 

 minutes. It has been found advantageous to add one per 

 cent of salt. After neutralization with soda, and further 

 steaming, filter the potato-agar into test tubes and steri- 

 lize once more. 



Blood Crystals. — A practical way for obtaining crys- 



