NATURAL HISTORY OF THE DIATOMACE^. 49! 



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 bubbles 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 one s self. 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 for small and delicately marked forms, on which very high power 

 objectives 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 clean water. The thinner kinds of glass are rather 

 difficult to clean ; 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 cam 

 bric handkerchief. The covers, always round, should be separated into 

 sizes and thicknesses, so that the exact kind of cover required can be 

 found without having to search for it by turning over a number, scratch 

 ing or breaking them, and losing much valuable time. We shall also 

 require a pair of forceps for holding the slides over the lamp ; and such 

 as are sold at house-furnishing stores and by grocers, under the name of 

 American clothes-pegs, and which have been already mentioned, are by 

 far the best I have ever seen or heard of. A small pair of brass forceps 

 which close with a spring will be needed, and they are best set in a 

 wooden handle so as to protect the fingers from the heat ; and another 

 pair, which spring open and may be closed by means of the finger and 

 thumb, will be wanted for taking hold of and adjusting the thin covers. 

 I do not advocate the use of paper covers for slides, but labels of some 

 kind will, of course, be required, and I have found the plain circular white 

 ones to look the best. There are very pretty square labels sold by dealers 



