360 



WELLS'S NATURAL PHILOSOPHY. 



SECTION V. 



Describe the 

 portable cam- 

 era obscura. 



Fig. 291, 



OPTICAL INSTRUMENTS, 



717. The portable camera obscura, such as is ordinarily 

 used for photographic purposes, consists of a pair of achro- 

 matic double convex lenses, set in a brass mounting (see Fig, 

 291), into a box consisting of two parts, one of which 



,^i| slides within the otlier. The total length of the box la 

 wTt adjusted to suit the focal distance of the lens. In the 

 |i-l back of the box, which can be opened, there is a square 

 ^^ piece of ground glass which receives the images of the 



-^ objects to which the lens is directed, and hy slidixig the 

 movable part of the box in or out, the 



ground glass can be brought to the ^IG- 292. 



precise focus. The interior of the box 

 is blackened all over to extinguish 

 any stray light. 



The appearance of the camera as 

 described is represented by Fig. 292. 



What are Spec- 718. SpGCta- 



'^''^^'- cles consist of 

 two glass or crystal lenses, 

 of such a character as to 



remedy the defects of vision in imperfect eyes, — mounted 

 in a frame so as to be conveniently supported before the 

 eyes. 



Spectacles are of two kinds, namely those 

 with convex glasses, which magnify objects, 

 or bring their images nearer to the eyes ; 

 and those with concave glasses, which diminish the ap- 

 parent size of objects, or extend the Hmits of distinct 

 vision. 



Some persons, in order to protect the eye from excessive light, use blue 

 glasses as spectacles ; they are, however, more mischievous than useful, since 

 they absorb different parts of the spectrum unequally, and transmit the violet 

 and blue rays. 



What is a Mi- ^^^- ^ Microscopc is any instrument which 



croscope? magnifies the images of minute objects, and 



enables us to see them with greater distinctness. This 



result is produced by enlarging the angle of vision under 



What are the 

 two varieties 

 of spectacles ? 



