Theory of Diffraction-gratings. 83 



above calculation, being founded on the supposition of entire 

 freedom from aberration, both spherical and chromatic, is sub- 

 ject in practice to a large modification. In astronomical tele- 

 scopes, where every thing is sacrificed to the requirement of ex- 

 treme definition at the centre of the field, the theoretical limit 

 is sometimes closely approached; but the case is very different 

 with a photographic lens. In fact the very first thing it occurs 

 to a photographer to do, when he wishes to improve the definition, 

 is to contract the aperture of his lens by means of a stop — a 

 course which would be attended with the opposite result in the 

 case of a perfect object-glass, or even a good astronomical tele- 

 scope. While, therefore, it might be too much to say that the 

 reproduction of 3000 lines in an inch by lens and camera is im- 

 possible, the attempt to do so without very special appliances 

 appears in a high degree unpromising. It would certainly 

 require a lens more than usually free from spherical aberration, 

 and unlike either a telescopic or a photographic object-glass*, 

 achromatic (if the expression may be allowed) for the chemical 

 rays, unless indeed the latter requirement could be evaded by 

 using approximately homogeneous light. It must be under- 

 stood that nothing is here said against the practicability of co- 

 vering a small space with lines at the rate o/3000 to the inch, 

 a feat probably well within the powers of a good microscopic 

 object-glass. 



The method of contact-printing, on the other hand, is free 

 from optical difficulties. The photographic film prepared on a 

 flat piece of glass (or other support) may be brought by mode- 

 rate pressure in a printing-frame within a very short distance of 

 the lines of the original grating; and if the source of light be 

 moderately small and the rays fall perpendicularly, the copy 

 rarely fails in definition, unless through some photographic de- 

 fect. When direct processes not depending on development are- 

 employed, the unclouded light of the sun is necessary. To avoid 

 too much diffused light, I usually place the printing-frame on 

 the floor of a room into which the sun shines, and adjust its 

 position until the light reflected from the plate-glass front is 

 sent back approximately in the direction of the sun. Too much 

 time should not be lost in this operation, which requires no par- 

 ticular precision. Usually I cut off part of the extraneous light 

 by partially closing the shutters; but I cannot say whether this 

 makes any difference in the result. Those who are accustomed 

 to this kind of experimenting will know that it is often less 

 trouble to take a precaution than to find out whether it is really 



* Photographic lenses are corrected on the principle of making the 

 " visual and chemical foci " coincident, which leads to a different construc- 

 tion from what would be adopted were the chemical ravs alone attended to. 



G 2 



