22 THE ART OF PROJECTING. 



the window falls perpendicularly upon it ; then slowly 

 move the lens from the paper toward the window, until 

 the inverted image of some object out of doors, such as 

 a cloud, or house, or tree, half a mile or more away, 

 appears plainest. Measure the distance from the lens 

 to the paper, as in the other cases. 



If diverging rays had been used instead of parallel 

 rays, the focus would have been at some greater dis- 

 tance ; and if converging rays, a less distance than that 

 indicated for parallel rays, and these differing foci may 

 be infinite in number ; hence, the focal length of a lens 

 is always specified for parallel rays. 



CARE OF LENSES. 



Before using lenses, or other pieces of glass appara- 

 tus, see to it that they are clean, and, as far as possible, 

 avoid touching them upon their polished surfaces with 

 the fingers, as the latter will always leave a mark upon 

 such a surface. A piece of cotton-flannel, or old fluffy 

 linen will do to wipe glasses with. Wet the cloth with 

 water, or, still better, with alcohol, if there are spots 

 that will not otherwise come off. One may know when 

 a piece of glass is clean by gently breathing upon it, 

 and then noticing how long the condensed moisture 

 remains upon its surface. If it is really clean, the 

 moisture will disappear in a second or two ; if it 

 remains for eight or ten seconds or longer, the glass is 

 not clean, though it may appear to be so to the eye. 

 Be careful to keep such pieces from touching anything 

 harder than the cloth they are wiped with, for even 

 wood will scratch a nicely polished glass surface. If 

 these pieces are not mounted in such a way as to pro- 

 tect them, they should be laid, when not in use, upon a 

 piece of cotton-flannel or velvet. 



