174 On the Photographic Camera Obscura. 



Instead of using a single lens, A B, with the concave side 

 towards the radiant object G ; let a second, and similar lens, 

 C D, be placed before it, with the coiivex side outwards, and 

 at a distance equal to one-third of its focus. 



That such a combination will be, in a great degree, free 

 from the aberration caused by the oblique rays, coming from 

 the extreme parts of the picture, will be evident upon tracing 

 the course of an oblique pencil, from its radiant point G, to the 

 image of that point at H. 



Of all the rays which fall upon the lens C D from the point 

 G, those alone which fall between C and the centre of the 

 lens are transmitted to H ; these fall at a comparatively high, 

 and equal, angle of incidence, upon both lenses ; through which 

 they pass nearly at right angles : while the more oblique rays 

 (which would ruin the picture), falling between the centre of 

 the lens and D, are all intercepted by the diaphragm E F. 



But besides preventing much of the aberration of the ob- 

 lique rays, the same combination has a tendency to correct 

 the chromatic error, as will appear by observing the course 

 of a single oblique ray G C. 



Fig. 2. 



i_ 



At the point C it will suffer dispersion, the red portion 

 being refracted to R, and the violet to V. If there were no 

 second lens, the rays would go on diverging and damaging the 

 picture : but, being received upon the second lens, at unequal 

 distances from its centre, the violet ray being furthest, it will 

 receive a stronger refraction ; and being besides more refran- 

 gible than the red, the spectral colours will now have a ten- 

 dency to approximate, to reunite, and be neutralized. 



It is scarcely necessary to show that the second lens will 

 increase the intensity of the light; if there were only one lens, 

 the focus would be at y (fig. I); and the arrow at y would 

 represent the size of the image ; but the second lens being in- 

 terposed, the convergency of the rays is quickened, and the 

 image will be formed at I, of the size there represented. The 

 same quantity of light is there condensed into about one-third 

 of the area, and the photographic power is increased in the 

 reverse proportion. 



With the same aperture, the two lenses define nearly as well 

 as when used singly : but, the light gained being very consider- 



