1897-98-] On the Diatomacece. 367 



One by one they are then picked up on the point of the 

 bristle, and transferred to position on the shellac on the 

 cover-glass. It will facilitate their placing on the shellac if 

 it is slightly moistened with a solution of petroleum and 

 benzole. When the design is completed the cover-glass is 

 removed with the utmost care — preferably by gently pushing 

 always on the same plane — to the mounting-plate, which is 

 slowly heated by the spirit-lamp. The shellac is softened, and 

 the diatoms placed on it adhere. When cool they are 

 mounted in balsam or styrax on a slide in the usual way. 

 The whole process is very trying, especially if elaborate 

 designs are attempted, and I would not advise it to be long 

 continued. 



Examination under the Microscope. 



The mounting is the last stage preparatory to the examina- 

 tion of the diatoms under the microscope. To provide object- 

 glasses for this examination the highest skill of the optician 

 has been called into requisition, and it is not too much to say 

 that the study of diatoms, more than any other single class of 

 objects, has been the cause of calling into existence the present 

 high class object-glasses. A prepared slide is placed on the 

 stage of the microscope, the lamp is arranged, and if this can 

 be done so that the flame will shine directly, without the 

 intervention of the mirror, into the sub- stage condenser, so 

 much the better. When a sharp bright light is required it 

 is best to use the narrow edge of the flame. The sub-stage 

 condenser is now focussed so that a sharp image of the lamp 

 flame is projected upon the object. An object-glass of 

 power and aperture sufficient to catch up the details of the 

 diatom is put on the microscope, and the examination is begun. 

 In this examination do not be discouraged although things do 

 not come up to your expectation. Beginners usually have 

 exaggerated notions of what is to be seen under the micro- 

 scope. When we consider the great range of difference in the 

 fineness of the striation of diatoms, we will easily understand 

 there are difficulties to be overcome before all the details are 

 seen. For instance, a Navicula lata, with its robust costse 

 of 7000 to 8000 in the inch, is easily resolved with an 

 object-glass of numerical aperture of - 30 ; while the Navicula 

 rhomboides, with its 85,000 stria? to the inch, requires an aper- 



