OF MICROSCOPIC OBJECTS. 37 



employed, the outer ring of coloured varnish may be applied as 

 elsewhere described and the slide finished. 



The diatoms are also sometimes mounted betwixt two thin 

 glasses, as described in Chapter I., so that the light by which 

 they are examined may receive as little interference as possible, 

 and that an achromatic condenser may be brought into focus 

 under the slide. 



Of the various modes of cleaning and mounting the Diato- 

 maceae, I believe that the following may be safely recommended 

 as affording results of the very best quality. My friend, Mr. T. G. 

 Rylands, gave it to me as that which he prefers, and I can safely 

 say that his numerous slides are at least equal to any I have ever 

 seen. I will give it just as I received it from him, though there 

 may be some little repetition of what has been said elsewhere, as 

 he does not appropriate any part of it as his own. He says : 

 In this branch of mounting, general rules alone can be laid 

 down, because the gatherings may contain iron, lime, fine silt, 

 or vegetable matter under conditions for special treatment, and 

 consequently the first step should be to experiment on various 

 kinds. 



In gathering diatoms much labour is saved by judgment and 

 care ; hence it is desirable to get acquainted with them in their 

 growing condition, so that when recognised upon the sands or 

 other spots they may be carefully removed by the aid of the 

 spoon or small tin scoop before described. When growing upon 

 alga3 or other plants, the plants and diatoms together may be car- 

 ried home, in which case they must be simply drained and not 

 washed or pressed, in order that the diatoms be not lost. As it 

 is always desirable to examine the gathering on the ground, a 

 "Gairdner's hand microscope" with powers from 80 to 200 

 diameters will be found very useful. The best gatherings are 

 those which represent one species abundantly. Those which are 

 mixed may be rejected, unless they are seen to contain something 

 valuable or important, as the object should not be so much to 

 supply microscopical curiosities as to collect material which is 

 available for the study of nature. 



The gathering when carried home should always be carefully 

 examined before anything is done with it ; not only on account 



