6 Marvels of Pond-Life. 



one hundred and eighty respectively; and with 

 a fifth two hundred and forty, four hundred and 

 thirty, and seven hundred and twenty, which can be 

 augmented by the use of a draw tube. 



It has been well observed that the illumination of 

 objects is quite as important as the glasses that are 

 employed, and the most experienced microscopists have 

 never done learning in this matter. Most microscopes 

 are furnished with two mirrors beneath the stage, one 

 plane and one concave. The first will throw a few 

 parallel rays through any transparent object properly 

 placed, and the latter causes a number of rays to 

 converge, producing a more powerful effect. The first 

 is usually used in daylight, when the instrument is 

 near a window (one with a north aspect, out of direct 

 sunlight, being the best) ; and the second is often 

 useful when the source of illumination is a candle or a 

 lamp. By varying the angle of the mirror the light is 

 thrown through the object more or less obliquely, and 

 its quantity should never be sufficient to pain the eye. 

 Few objects are seen to the best advantage with a 

 large pencil of perfectly direct light, and the beginner 

 should practise till the amount of inclination is obtained 

 which produces the best effect. 



It is advisable that the hole in the stage of 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 arm 

 by which it can be thrown completely out of the 

 perpendicular plane of the body of the instrument. 



