234 THE AMERICAN MONTHLY [Au 



to contain the object. This method of mounting opaque 

 objects upon a black background is not only unnecessary, 

 but often inconvenient, as it renders the use of transmit- 

 ted light impossible, if it should be wanted ; neither can 

 such slides be examined by means of a Lieberkuhn. We 

 recommend therefore that the black background be omit- 

 ted, and that instead a similar disk, or two or three disks 

 of various sizes, be put upon thin slips, and one of these 

 can then be placed beneath the slide carrying the object, 

 when it is being examined by reflected light. 



Cells. — The cells are made by running a ring of gold- 

 size of the same diameter as the cover-glass that will be 

 used. This is done by means of the turntable, and is not 

 difficult. It is not advisable to use too full a brush, and 

 the gold-size should be of the right consistency — neither 

 too thick to leave the brush, nor so thin as to run away 

 from position. The tip of the brush is used, and the ta- 

 ble rotated not to quickly. For very thin objects one ring 

 will suffice ; but thicker objects will need two or three rings, 

 added one on the top of another, each ring being added, 

 however, only when the other is dry. If a few such rings 

 do not give sufficient depth, it is advisable to build up the 

 cell by other means. Rings may be cut out of stout paper 

 or thin and good cardboard, then steeped in parafine and 

 dried. Stout rings of ebonite, glass, tin, etc., can be ob- 

 tained from the opticians. It is only necessary to attach 

 these to the slide by means of a ring of gold-size, press- 

 ing down the ring firmly, and even giving a very slight 

 twisting motion to make sure of there being no air-bub- 

 bles to prevent perfect contact. If the cells of gold-size 

 when dry should not be quite level, they can easily be 

 rubbed down on a piece of very fine emery laid on a flat 

 surface. The object itself must be fastened in place by 

 means of a drop of gum placed upon the slide. Care 

 must be taken that this drop of gum is hidden by the ob- 

 ject, unless that is impossible. Thin objects, such as 



