WITH THE MICROSCOPE. 19 



piece, pi. XIII, figs. 57 and 58. Either the analyser ex the polarizer 

 should be so arranged that it may be made to rotate. ' 



By polarised light the internal structure of various transparent 

 objects can be rendered evident in a very beautiful manner, but for 

 ordinary microscopical work this method of observation is of little 

 use, and I think the advantage of polarised light in general micro- 

 scopical enquiries has been much overrated. In examining objects 

 by polarised light beautiful effects may be obtained by interposing 

 between the polariser and the object thin plates of certain crystalline 

 substances which should be so arranged as to be capable of revolving. 

 The play of colours which may be produced in this way by the aid 

 of selenite is in the case of many objects very beautiful. Plates of 

 different degrees of thickness, each giving a different colour, may be 

 obtained of the opticians. 



Sources of Illumination. 



Ordinary daylight or sunlight reflected from a white cloud affords 

 the best illumination, but the light of a candle, gas, or good lamp 

 answers exceedingly well if certain precautions be taken. Daylight 

 is usually reflected from the mirror. In the examination of trans- 

 parent objects the microscope is arranged as in pi. X, fig. 42, and the 

 light is usually reflected by the mirror. Sunlight is only employed 

 under very special circumstances, as for examining objects by 

 coloured media, when an intense light is required, or for the pur- 

 pose of taking photographs of microscopic objects. See Part IV 

 on Photography. 



24. Artificial illumination. It has been said with truth that 

 microscopical work should be undertaken only by day, since the 

 most perfect artificial light which can be obtained is far inferior to 

 daylight for delicate observation, while it strains the eyes very 

 much more. But unfortunately it happens that in this country, 

 especially in our large cities, during a great part of the year, our day- 

 light is not very suitable for microscopical investigation, while some 

 of us, in consequence of being occupied in work of perhaps a very 

 different kind by day, are compelled to work principally or entirely by 

 ni"ht. It is therefore a matter of the greatest importance that we 

 should be provided with a good kind of artificial illumination. 



Lamps. 



From time to time various microscope lamps have been pro- 

 posed. The small catnphine lamp brought out many years ago by 



c 2 



