DECORATIVE NATURAL HISTORY WORK. 113 



exercise of any particular skill. In the first place, the 

 quantity of gelatine required is so small that the 

 overturning of the stock solution would not cause any 

 appreciable damage. The quantity recommended in 

 the textbooks would coat something like a square mile 

 of glass. In the second place, the best way of coating 

 glass plates is to take a small wad of cotton-wool, dip 

 it in the gelatine solution, squeeze it out, and go gently 



Chameleon. 



over the glass backwards and forwards, until it is com- 

 pletely covered with an exceedingly thin coating. How 

 thin this coating is can be realised when the gelatine is 

 dry. It will then be found and this is a good test of 

 proper coating that the glass will be covered with 

 iridescent bands, due to interference and following the 

 original motion of the cotton-wool over its surface. 

 When bichromate of potash has been used the coated 

 plate should be exposed to bright sunshine for about 



