PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 915 



used ill the following manner: (Here is inserted a small engraving 

 illustrating his reflecting camera, which is like one form of the 

 reflecting telescope.) 



The person is placed at A, and in the axis of the reflector B. 

 C is the plate on which the ifnage is to be formed. D D are rays 

 radiated from the person and falling on the mirror B. E E are 

 the same rays converging to a focus after reflector C is inclosed 

 in a box to exclude the extraneous light. 



From experiments which I have made, I find that a speculum, 

 of 7 inches aperture and twelve inches focus, will form a picture 

 in about the same time as a single lens of 3| inches aperture and 

 12 inches focus. Taking this as a standard of comparison, we 

 may make an estimate of the action of lenses and reflectors. 



In consequence of a lens inverting the image of an object, it 

 will be necessary to use a flat speculum, in order to cross the rays; 

 and, to reduce the aberrations, the lens should be a compound 

 one, consisting of at least three glasses. If we estimate the rays 

 lost by reflection from each surface as jL ^^^ ^^^ ^^J^ transmitted 



by a single lens as 100.00 



we have, loss by 1st surface of 2d glass 8.33 



91.67 

 Loss b}'^ 2d surface of 2d glass 7.63 



84.04 

 Loss by 1st surface of 3d glass 7.00 



77.04 

 Loss by 2d surface of 3d glass 6.42 



70.62 

 Loss by flat speculum | 26.48 



Rays transmitted after one reflection, and refraction through 

 three lenses, as compared with that transmitted through 

 one lens of the same aperture 44. 14 



Now as 44 : 100 so is 100 : 227; that is if the area of the 

 single lens is 100, that of the triple lens must be 227 ; and taking 

 the square roots of the number for the apertures, we have \/100 : 

 v/227, so is 3.66, the aperture of the single lens, to 5.49 that of 

 the compound oue. With a lens, we are under the disagreeable 

 necessity of placing it about -^^ of its focal distance nearer the 



