OF THE MICROSCOPE. 33 



between the lens and the flat surface is exactly equal to the 

 focal distance of the lens. Transparent objects, such as the 

 scales of insects, animalculse in water, etc., are simply placed 

 on the flat surface of the glass cylinder, and when looked at 

 through it, they appear greatly magnified. It is easily used, 

 but can not well be employed as a working microscope. It is 

 this kind of lens that is used in the construction of those watch 

 charms in which a large picture is seen on looking through a 

 very small hole. The picture is a photograph attached to the 

 flat end of a small glass rod, the other end of the rod being 

 formed into a lens of exactly the right focal length required to 

 show the picture clearly and considerably magnified. Lenses 

 and photographs of this kind are usually mounted as miniature 

 opera-glasses. 



The Stanhope lens seems to be a favorite in France, where it 

 is manufactured very extensively, and sold under the name of 

 the Stanhoscope. One advantage claimed for it is that no adjust- 

 ment for focus is required, the flat surface of the lens being 



c 



Fig. 8. STANHOPE COLLECTING LENS. 



exactly in the focus of the curved surface. This feature, while 

 presenting some slight advantages, has also the great disad- 

 vantage that objects covered with thin glass cannot be ex- 

 amined by such lenses, and neither can objects having a 

 sensible thickness. The feature which we have just mentioned 

 is imitated in some cases by placing a piece of thin glass in 

 front of a small lens, and at such a distance that the outer sur- 

 face of the thin glass will be exactly in focus, as is hereafter 

 described when speaking of the Craig microscope. 



There is one purpose, however, to which this form of lens is 

 applied with good effect, and that is to the construction of a 

 " collecting lens " as it is called that is, a lens for examining 

 drops of water when searching for diatoms, algae, animalculse, 

 etc. Fig. 8 shows a lens of this kind made by Jas. W. Queen 

 & Co., of Philadelphia. The lens is set in a brass frame, pro- 



