30 SELECTION AND USE 



other end is thrust through the second hole in the cork and is 

 supported by it. 



Whenever a piece of apparatus is to be supported steadily, 

 while at the same time it is necessary that it should be easily 

 moved and adjusted, nothing serves so well as a fine cork 

 sliding on a smooth wire. 



Twenty-five cent Microscopes. Before leaving this 

 subject it may be well to say a few words about those very cheap 

 microscopes which have been so extensively advertised. We 

 frequently see in the papers an advertisement in which some 

 person offers to send for twenty-five cents a microscope which 

 will show animalcules in water, globules of blood etc., etc., and 

 the question naturally arises, Are these microscopes good for 

 anything, or is the advertisement a swindle the advertiser 

 taking the money and sending nothing in return? 



As a general rule, those who send to such advertisers, receive 

 in return, a plate of brass or lead, with a glass bead fastened 

 in a hole in the centre. The glass bead is formed by fusion and 

 is frequently ground flat and polished on the side by which it 

 was attached to the thread or rod of glass from which it was 

 made, forming in such cases a hemispherical lens. Such 

 lenses are very easily made by any one. Take a strip of flint 

 glass, such as a piece of flint glass tubing, or a piece of glass 

 rod, draw it out to a thread in the flame of a spirit lamp, 

 fuse the end and allow it to gather into a drop. Give plenty of 

 time and a good strong heat, so that the surface of the little 

 globe may become well-fused and truly round. The best re- 

 sults are always obtained by holding the thread perpendicularly, 

 as when held horizontally the globule is apt to become dis- 

 torted. Make one or two dozen of these, and in separating 

 them from the glass rod leave about an eighth of an inch of 

 the latter attached to each globule, to serve as a handle, in the 

 next step of the process, which consists in inserting them to 

 about half their depth in a plate of cement, consisting of 

 shellac thickened with very dry and finely powdered pumice- 

 stone. To form such a wax plate, melt some shellac in a ladle 

 or large iron spoon, mix it carefully with as much powdered 

 pumice-stone as can be conveniently stirred in, remove it from 



