THE MICROSCOPE. 27 



parallel lines from mirror ; d, the same rays made 

 to rapidly converge by passing through the lens, 

 and come to a focus at e ; and if the focal length 

 of the objective is greater than the distance between 

 the object and the point e, the object will be illu- 

 minated, and the field appear perfectly dark. 



Fig. lie. 



a, " Spot Lens," front view ; c, blackened concavity of 

 ditto ; a', section of " Spot Lens " in its fitting, b ; c 7 , central 

 stop ; d d, parallel rays of light converging to a focus at e. 



Having said thus much with regard to apparatus, 

 we will now give some directions for the use of 

 the Microscope under ordinary circumstances. The 

 Microscope may be either used by the light of the 

 sun in the daytime, or at night by some form of 

 artificial light. It is best used by daylight, as 

 artificial light is likely to tire the eyes. 



Having determined to work by daylight, some 

 spot should be selected near a window, out of the 



