EDUCATIONAL VALUE OF PHOTOMICROGRAPHY. 147 



successfully a series of such lights, beginning with a two-wick oil lamp 

 behind a large Florence flask filled with water, the flask serving at once 

 as a condenser and heat filter. Afterward, in succession, was 

 used an argand gas lamp; a Welsbach burner; a three-wick pro- 

 jection lantern, burning camphorated oil; a 50-candle power incan- 

 descent lamp; and a four-burner acetylene lamp with stereopticon 

 double condenser, to the final 

 and most efficient light of all, 

 an arc light which can be 

 regulated between 1,000 and 

 5,000 candle power. Of course 

 the arc light gives out a great 

 amount of heat as well as a 

 satisfactory quantity of light, 

 and were the rays to fall di- 

 rectly upon the microscope 

 lenses the temperature would 

 rise high enough to endanger 

 their mountings. To obviate 

 this difficulty a distilled water 

 cell two inches thick is placed 

 in front of the stereopticon 

 double condenser, which is 

 directly before the arc at such 

 a distance as to parallelize the 

 divergent beam. It is a com- 

 mon idea that an alum solu- 

 tion is the best for absorbing 

 heat while transmitting light. 

 That such is not the case is 

 proved by experiment. Mel- 

 lcni and others have shown 

 that distilled water will in- 

 tercept more heat rays than a 



solution of alum. The writer has verified that conclusion and is also 

 able to show that distilled water transmits more light, has practically 

 the same heat-absorbing power through a considerable range of tem- 

 perature and also has the advantage of no formation of crystals in the 

 cell. Distilled water is necessary, as with common water the air 

 bubbles collect on the parallel faces of the cell as the temperature 

 rises, and obstruct the passage of light. 



The form of apparatus shown complete in Fig. 1, using a Zeiss 

 microscope and Bausch & Lomb adjustable camera with automatic 

 regulating arc light, gives excellent results for ordinary dry mounted 



Fin. 2. Apparatus Complete for making In- 

 stantaneous Pictures of Living Microscopic 

 Animals. 



