38 Photography for the Sportsman Naturalist 



A lens does not form the image ; the rays of 

 light do that, and a pinhole will allow the rays of 

 light to pass as well as will a lens. The lens, 

 however, gathers together these rays and sifts 

 them, discarding those that are not needed and 

 allowing only those to pass which are necessary 

 to the formation of the picture. 



If we wish to understand thoroughly the prin- 

 ciples of a lens, and how it does its work, we 

 must obtain some book, of which there are many 

 published, upon this subject and read it, for here 

 I shall only touch upon the more important points. 

 In fact, I should advise the beginner to study some 

 elementary work on optics as applied to photog- 

 raphy, and also the elements of photographic chem- 

 istry. He will find that the knowledge he will 

 thus gain will cause him to work to much better 

 advantage, and the time spent in studying these 

 things will not by any means be lost. 



The modern double lens is made in two com- 

 binations, that is, there are two sets of glass 

 used in its construction, and, therefore, it is what 

 is termed " convertible," i.e. each combination 

 can be used separately, or the two can be used 

 together, thus making, in reality, three lenses in 

 one. 



The " focal length " of a lens is, roughly speak- 

 ing, the distance between it and the ground glass 

 when an object at a distance is in focus, or, in 



