206 OPTICAL PROJECTION 



by pins, passed through the loops so as to stretch out the 

 threads and consequently the toes, with more facility than in 

 any other way, and the stage clips hold the rough cork more 

 securely than polished brass. One hint is however necessary, 

 which applies to any other object, where a moist surface is 

 involved ; and the frog's foot must be moist, and the frog 

 kept damp in his bag. The objective must be warmed, else 

 the moisture will condense upon it, and baffle all attempts at 

 an image. It may seem ridiculous to mention so simple and 

 obvious a precaution ; but having myself been fairly beaten 

 by such an oversight on one occasion, I know how easy it is 

 for such precautions to be overlooked or forgotten. 



Enough has now been said to indicate the range of power 

 of the oxy-hydrogen form of the instrument. Beyond a 

 certain point, the light fails us, or even the definition from so 

 large a radiant as we must employ. With class-room dis- 

 tances averaging 12 feet, and fine histological slides, powers as 

 high as T 1 ^ immersion may be used even with this light, with 

 or without an amplifier or eye-piece ; on the other hand only 

 a few of the coarsest diatoms will exhibit sufficient distinct- 

 ness, there being no real opacity in the markings. But with 

 general opacity all over a slide the power available decreases ; 

 and as a rule or 1 inch, more or less amplified, is the 

 highest power useful with this form of the instrument. Even 

 such a lens can be pushed on some objects, which combine 

 general transparency with opacity of detail, up to 2,500 

 diameters or more. 



103. The Electric Microscope. By employing the electric 

 arc, the power of the projecting microscope is enormously 

 increased, definition being improved, as already explained, 

 together with illumination. 



The general arrangements of an instrument constructed 

 for an arc-light do not differ from those already described. 

 But I found one alteration in detail to be absolutely necessary, 

 and another advisable. As regards the first, much more 



