51 



boiling the disks for a considerable time in nitric acid, and then 

 carefully washing with distilled water, but it is only here and there 

 that the separation is effected, and still less frequently that the sili- 

 ceous frame-work is obtained entire ; but that each disk really con- 

 sists of these two portions is easy demonstrable by the following 

 mode of manipulation. Select as large a disk as you can meet with, 

 and with a very fine sable pencil place it on a convenient black 

 ground, and. place this under the microscope to be viewed as an 

 opaque object. If a T 4 oth inch objective be used, a side light will an- 

 swer very well. Now, on inspecting the disk thus placed, it will 

 present to the eye the appearance of either the membrane (fig. 1), 

 or the frame-work (fig. 2) ; and if it be removed from the stage, 

 and carefully turned over by means of the sable pencil (which can 

 readily be done if the specimen selected be of a tolerable size), it 

 will then present the contrary appearance to that at first exhibited. 

 That these two portions are really distinct, is also readily shown 

 by focussing up and down, which will show each portion in a distinct 

 plane. 



As the method just described is only available for the larger spe- 

 cimens, and cannot be applied to mounted objects, I adopted the fol- 

 lowing manner of testing the same facts. Place a drop of water, 

 containing a few of the disks, on a square of thin glass, such as 

 is used to cover objects with, and when the water has evaporated, so 

 as to leave the objects dry, they will adhere to the glass; it is then 

 requisite to cover the objects with a similar piece of thin glass, and 

 mount the whole in the manner devised by Mr. Gillet, that is, 

 by attaching the two pieces of glass, with the object included, to a 

 slip of wood or other material, having as large a hole in it as the 

 size of the slide will allow. The slide is then to be placed on the 

 stage of the microscope, and viewed as a transparent object with 

 the achromatic condenser attached. Having selected any individual 

 to examine, produce the smallest possible spot of light by adjusting 

 the achromatic condenser, then turn back the body of the instru- 

 ment, and with a fine camel's-hair pencil, with a little carmine or 

 Indian ink, draw around the spot of light a light line, so as to in- 

 sulate the particular individual selected. By varying the form of 

 the insulating line, such as drawing a circle around one, a triangle, a 

 square, or a diamond around others, several on the same slide can 

 be particularised. The achromatic condenser is then to be removed, 

 and the smallest of the dark wells put in its place. The lieberkuhn 



