L HEAD ii'W USDS. 23 



hot evaporating-dishes with. Then there will be required a few 

 dip-tubes, made of small glass tube, drawn out over a ftam< 

 that the opening is considerably diminished. Tin* mode of 



making these cannot be given here, but will be found in b 

 OU chemical manipulation; and it will be well for the student 

 to learn to make his own dip-tubes, as a number will be required 

 first and last, and they are easily broken. Of course there will 

 be required a number of glass slides, of the usual dimensions 

 of three inches by one. These should be of as white glasf 

 possible, and it will be found best to procure those with ground 

 edges, as they are the neatest in appearance. Only such as are 

 free from scratches or other blemishes in the central square 

 inch should be used; and, although even such as have bub 

 or scratches near the ends only will not look ornamental in a 

 cabinet, we should remember that microscopic objects are not 

 generally mounted to look well in a cabinet, but to be useful 

 out of it; so that if the central and useful portion of the slide 

 be perfect it need not be rejected. Some persons make their 

 own glass slides, but I have never found it answer to do so, as 

 it is difficult to get the right kind of glass, not at all easy to 

 cut it or grind the edges, and it is liable to be scratched while 

 cutting or grinding. Thin glass, such as is made on purpose 

 for microscopic use, will be required; and this, also, it will be 

 found best to buy ready cut rather than attempt to cut it for 

 oneself. The thin glass used for covers may be of different 

 thicknesses, but the thickest made will not do for diatoms, and 

 a certain amount of the very thinnest will be required lor small 

 and delicately marked forms, on which very high power objec- 

 tives will have to be used. The covers must be perfectly clean, 

 which may be insured by soaking in caustic potassa solution, 

 and then washing thoroughly in dean water. The thinner 

 kinds of glass are rather difficult to elean; but with a little 

 extra caution it may be accomplished, the last polish being given 

 to it by a piece of an old and well-worn cambric handkerchief. 

 The covers, always round, should be separated into sizes and 



thicknesses, so that the exact kind o( cover required can be 



found without having to search tor it by turning 0V< I a num- 

 ber, scratching or breaking them, and losing much valuable 



time. We shall also require a pair of forceps for holding the 



