A Manual Of Photography

182 SCIENTIFIC INVESTIGATIONS ON PHOTOGRAPHY. to fit it a finît glass lens, the refractory power of which would place the foci of the rays at e, d, it will he seen that the result of such a combination would be the formation of a colourless image, at a mean point between them, by recombining the rays into white light. Such as is represented in the figure is the achromatic lens of a camera obscura. There is, however, a point to be examined in connection with the lens for photographic purposes, which is of the first im- portance, and which has not hitherto been sufficiently attended to. It is this. The luminous and coloured rays of the spectrum, and the chemical rays, are not coincident at any point of the spectral image, and the relation between the chemical power, and the illuminating power, of a ray, is subject to constant variations. It is often stated that the violet and blue rays are the chemical rays, and hence it is inferred, if the glass of a camera is corrected so as to malte these rays, and the less refrangible red, to correspond, all is done which can be desired. It must be distinctly understood that the colour of any par- ticular ray has no direct relation to its chemical character. It is true, if the more refrangible rays are made to correspond with the more luminous rays, we approach the desired point, but we do not necessarily reach it. It has been said we may do so by overcorrecting a lens ; but this is not the case, since beyond the limits of the chemical rays we have rays which have decidedly a protecting action, and if these are thrown into the field, the operation is retarded. We commonly hear of a lens being slow or quick ; this is purely accidental, arising entirely from the uncertainty in which all our optical instrument-makers remain as to the rela- tion of the chemical and luminous forces to each other. If the lenticular correction reaches and does not exceed the point of bringing the rays beyond Fraunhofer’s line H, upon the field of vision, the lens will be a quick one, as it is called. On the contrary, if it does not reach, or if it goes beyond this, it will be slow in action, because either the light rays interfere, as is explained in a previous page, or those rays beyond the chemical spectrum to which attention has of late been directed by the very refined researches of Mr. Stokes. ' For portraiture, and all purposes requiring great distinctness of outline and rapidity of operation, two achromatic lenses are usually employed. By this arrangement the focal distance is diminished; the image is much reduced in size, but then it is concentrated in every respect, and hence improved in all the necessary particulars. These lenses are, however, still open to