OPTICAL INSTRUMENTS. 173 



advantage of great magnifying power will be obtained, with 

 a field nearly equal to many of the compound microscopes. 

 The portability of this instrument, its low price, and the 

 facility with which it can be used, must recommend it 

 strongly to all who use microscopic lenses. With it may be 

 viewed the animalcules in water, mites in cheese, eels in 

 paste and vinegar, the perspiration, human hair, farina, and 

 leaves of flowers, the hairs of animals, the down of moths, 

 &c. ; and if a single drop of the crystallization of salts be 

 spread lightly over the end of the lens, and viewed without 

 delay, the formation of the crystal will be beautifully appa- 

 rent. Price, $2.00. 



Fig. 671. Fig. 672. 



Linen and Cloth Prover. (Figs. 671 and, 672.) This in- 

 strument for ascertaining the fineness of linens, muslins, 

 bolting cloths, &c. ; by ascertaining the number of threads 

 in a given space under a powerful magnifying glass, consists 

 of a lens mounted in a brass frame, about one inch over a 

 square hole that is exactly half, or quarter of an incli square, 

 and situated in the focus of tha lens. The hole is placed on 

 the linen, or fabric to be examined, and held in a position to 

 receive a strong light, when it will appear, on viewing 

 through the lens, magnified to such an extent that the 

 number of threads in the space may readily be counted, and 

 the fineness thereby accurately ascertained. This instru- 

 ment will answer the purpose of magnifying small objects, 

 placed on the top of the plate, equally well. They are 

 inclosed in small round boxes of wood or brass. 



Price, in wood box, - 63 cts. 



" in brass " 75 cts. 



" extra finish, 88 cts. 

 15* 



