1878.] 



of determining Ratio of Dispersions. 



487 



the telescope to a vertical line separating light from dark, such as the 

 edge of a chimney seen in the shade against the sky, and covering 

 half the object-glass with a screen having a vertical edge. So delicate 

 is this test that on testing different telescopes by well-known opticians, 

 a difference in the mode of achromatism may be detected. The best 

 results are said to be obtained when the secondary green is interme- 

 diate between green and yellow. This corresponds to making the 

 focal length a minimum for the brightest part of the spectrum. 



To enable me to form a judgment as to the sharpness of the test 

 furnished by the tint of the secondary green, as compared with the 

 performance of an object-glass, I tried the following experiment. 



A set of parallel lines of increasing fineness was ruled with ink on 

 a sheet of white paper, and a broader black object was laid on it as 

 well, parallel to the lines. The paper was placed, with the black lines 

 vertical, at a considerable distance in a lawn, and was viewed through 

 two opposed prisms, one of crown glass and the other of flint, of such 

 angles as nearly to achromatize each other in the positions of minimum 

 deviation, and then through a small telescope. The achromatism 

 was now effected, and varied in character, by moving one of the 

 prisms slightly in azimuth, and after each alteration the telescope was 

 focused afresh, to get the sharpest vision that could be had. I found 

 that the azimuth of the prism was fixed within decidedly narrower 

 limits by the condition that the secondary green should be of such or 

 such a tint, even though no attempt was made to determine the tint 

 otherwise than by memory, than by the condition that the vision of 

 the fine lines should be as sharp as possible. Now a small element of 

 a double object-glass may be regarded, so far as chromatic compensa- 

 tion is concerned, as a pair of opposed prisms ; and therefore we may 

 infer that the tint of the secondary green ought to be at the very 

 least as sharp a test of the goodness of the chromatic compensation as 

 the actual performance of the telescope. And such Mr. Thomas 

 Grubb, to whom I mentioned the test, found to be actually the case in 

 the progress of the construction of the 15-inch refractor for the Royal 

 Society, the instrument at present in the hands of Mr. Huggins. 



It follows therefore that two opposed prisms, representing the 

 glasses to be employed in an objective, are to be deemed to achromatize 

 each other when one of the two secondary colours is about midway 

 between yellow and green. 



The condition of achromatism of two opposed prisms is given in the 

 ordinary treatises on optics, but, so far as I have seen, rather as an 

 optical curiosity than as a matter of practical utility. Sir David 

 Brewster in his treatise on " 'New Philosophical Instruments," p. 292, 

 alludes to it as furnishing a conceivable mode of determining disper- 

 sive powers, but mentions it only to condemn it. I cannot imagine 

 why, for at least with the modifications which I have been led to in- 



