294* optics. 



situation be, provided it shine at all upon the 

 window. 



The following is a diagram of the solar micros- 

 cope: Let A B (Fig. 5.) be a section of the window- 

 shutter of a dark room, containing a scioptric ball 

 E F; in the fore-part of which is screwed the tube 

 G I K H, at one end of the tube is a lens G H, 

 which, by converging the sun-beams into a narrow 

 compass, strongly enlightens the small object a, 

 placed on a slip of glass, or otherwise, in the part 

 of the tube G H, where a slit is made on each 

 side for that purpose. Within this tube there 

 slides another, L m, which contains a small mag- 

 nifying lens, m r. 



By moving the exterior tube I G H K one way 

 and the other, the glass G H will be brought to 

 receive the rays of the sun directly, and will there- 

 fere most intensely illuminate the object a. The 

 other tube L M, being slid backwards and forwards, 

 will adjust the distance of the small lens, so that 

 the image of the object a shall be made very dis- 

 tinct on the opposite side of the room at O P; and 

 the magnitude of the image will be to that of the ob- 

 ject, as the distance from the lens mris to the distance 

 of the object from it; as is evident fromthe figure. 



A micrometer is an instrument for measuring 

 extremely small objects. It is of various construc- 

 tions. The following one is extremely simple. It 

 is nothing more than a stage on which the objects 

 are placed, moveable by a fine screw, which has a 

 hand, or small index, passing over the divisions of 

 a graduated circle. A fine threaded screw is the 

 essential part in all micrometers of the best or 

 most perfect kind. This screw was formerly placed 

 in the focus of the eye-glass, just where the image 

 is formed: but this method of applying it was 



