Marvels of Pond-Life. 7 



This enables such oblique rays to be employed as to 

 give a dark field, all the light which reaches the eye 

 being refracted by the object through which it is sent. 

 The opticians sell special pieces of apparatus for this 

 purpose, but though they are very useful, they do not 

 render it less desirable to have the mirror mounted as 

 described. 



Most microscopes are furnished with a revolving 

 diaphragm, with three holes, of different sizes, to 

 diminish the quantity of light that is admitted to the 

 object. This instrument is of some use, and offers a 

 ready means of obtaining a very soft agreeable light for 

 transparent objects, viewed with low powers. For this 

 purpose cut a circular disk of India or tissue paper, 

 rather larger than the biggest aperture ; scrape a few 

 little pieces of spermaceti, and place them upon it, then 

 put the whole on a piece of writing-paper, and hold it 

 a few inches above the flame of a candle, moving it 

 gently. If this is dexterously done, the spermaceti will 

 be melted without singeing the paper, and when it is 

 cold the disk will be found transparent. Place it over 

 the hole in the diaphragm, send the light through it, 

 and the result will be a very soft agreeable effect, well 

 suited for many purposes, such as viewing sections of 

 wood, insects mounted whole, after being rendered 

 transparent, many small water creatures, etc. Another 

 mode of accomplishing this purpose is to place a 

 similarly prepared disk of paper on the flat side of a 

 bull's-eye lens, and transmit the light of a lamp through 

 it. This plan may be used with higher powers, and 

 the white opaque light it gives may be directed 



