10 MARVELS OF POND-LIFE. 



the microscope should be large — at least an inch 

 and a half each way — so that the entrance of oblique 

 rays is not obstructed, and it is desirable that the 

 mirror, in addition to sliding up and down, should 

 have an ni'ni by which it can be thrown completely 

 out of tlie j)erpendicular plane of the body of the 

 instrument. 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 agree- 

 able liglit 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 Hame of a candle, moving it gently. If 

 this is dexterously done, the spermaceti will be 

 melted without singing the paper, and when it is 

 cold the disk will be found transparent. Place it 



