A Manual Of Photography

LENSES FOB TUE PHOTOGRAPHIC CAMERA. 181 colour. At one distance red will prevail, at another violet. This is the result of chromatic aberration, and arises from the unequal refrangibility of the dissimilar rays. The red ray is less hent than the violet ; consequently, supposing the rays R R to fall on the edge of a lens, they will converge to a point 35. at r, whereas if the rays v v fall along the same circular line, they will, being more refracted, meet at /. Now if we place a disc at e, just the size of the cone of light, it will be edged with violet, but if we move it to a, the coloured border will be red. The indices of refraction for the several rays have been most carefully determined by Fraunhofer, and for a standard medium, a flint glass prism, they are respectively Bed........1-627749 dark line B Orange . 1629681 ........C Yellow.... 1-635026........D Green .... 1'642024 ........E Blue......1-648260 .........F Indigo.... 1-660285 ........G Violet .... 1671062 ........H Fraunhofer has determined the absolute values from the fixed dark lines which he observed in the spectrum : they represent, however, very closely the rays distinguished by their colours. By referring to the table of the refractive powers of trans- parent bodies (page 178), it will be seen that for a beam of white light, the difference between the most refractory flint "lass and crown glass, in their refracting powers, is as 2'028 is to 1534, and tins proportion is maintained nearly, but not ewwtlv for all the coloured rays: if, therefore, we have a crown "lass lens the refractive power of which will place the focus ata,for the violet rays, and at b, for the red rays, and we grind 36.