38 SELECTION AND USE 



Penny Microscopes. A few years ago a man in London 

 made a living by selling through the streets a microscope which 

 would show the eels in paste and vinegar, and of which the 

 price was only one penny, (equal to two cents.) These micro- 

 scopes were thus made : In the bottom of a pill-box he punched 

 a small hole and then blackened the inside of the box. In this 

 hole was placed a drop of Canada balsam or damar varnish, 

 which was allowed to dry. When hard, the balsam formed a 

 very tolerable lens. 



A drop of water, balsam, or varnish, laid on the under side 

 of a slip of clear glass will often enable us to extemporize a 

 microscope capable of doing good service in the hands of a 

 skillful observer. The outline of the drop should be perfectly 

 round, and the glass plate should be held as level as possible. We 

 have derived great assistance from such a lens, when better 

 could not be had. 



The Craig Microscope. This microscope at one time 

 attained an unprecedented degree of popularity, not on account 

 of its merits, but because of the extensive puffing and adver- 

 tising which it received. It consists of a vertical frame, some- 

 what like that of the cheap French microscopes, liaviug a 

 mirror, but no sliding tube, as there is no occasion for any. The 

 slide which holds the object is slipped through a horizontal slit 

 cut in the stand, and the lens with its frame is laid on it. 



The lens is a fused bead of glass set in a little frame, to the 

 under side of which is attached a thin plate of glass, whose 

 lower surface is exactly in the focus of the bead, so that when 

 a drop of water or vinegar is placed on the glass plate, or such 

 objects as insects' scales, wings, etc., are laid on it, they are ex- 

 actly in focus. Hence, this microscope is said to require no 

 adjustment for focus. This is true when the objects to be ex- 

 amined are actually in contact with the glass plate, but when 

 we wish to examine objects that are covered with thin glass (as 

 all valuable preparations should be) or objects having a percep- 

 tible thickness, it is impossible to adjust it for focus, and hence 

 it is impossible to examine such objects satisfactorily. Besides 

 this, nine-tenths of the microscopes of this pattern in market, 

 are very badly made, and distort objects to such au extent that 



