Dr. Col Woochvard to the " Colour Test." 63 



like cloudiness. Dissipate this hij better spherical correciions, and 

 it is immediately replaced by all brilliant points becoming irradiated 

 with the orange-red halo. 



In the same paper (July, 1872) I further stated — 



" If the orange-red rays be absorbed by the ammonio-sulphate 

 of copper solution transmitting monochromatic hght, definition 

 actinically is much more easily produced. But the eye, receiving 

 ordinary compound light, whilst the aberration is neglected to the 

 advantage of the achromatism, cannot possibly see as well, as the 

 actinic monochromatic rays potentially define and depict. A point 

 may be ascertained in the axis where the aberration may almost be 

 extinguished when it is no longer confused with uncorrected red 

 and yellow rays. In other words, the blue rays may be brought 

 accurately to the same mathematical point in the axis, whilst all 

 other (coloured) rays would vary more or less. In the finest 

 objectives now made I find when the spherical aberration is cor- 

 rected there is a strong secondary spectrum, chiefly consisting of a 

 mixture of the red and yellow rays. But when this is corrected, 

 by using all possible precautions, so as to render the achromatism 

 almost absolutely perfect, then the aberration re-appears. In point 

 of fact, the focal point where the spherical aberration vanishes, does 

 not correspond, or is not identical with, the focal point when the 

 coloured rays are blended into white hght. But as all makers (and 

 indeed everyone else) more readily detect imperfect achromatism 

 than residuary aberration, the latter is sacrificed to the former. 

 All the glasses with which I am acquainted err in this respect, or 

 aberrate," p. 266. — ' Quarterly Journal of Microscopical Science,' 

 July, 1872. 



Dr. Woodward expresses himself forcibly in the same direction, 

 p. 299, No. xxvii., 1872 :— 



" I have observed that those glasses which were quite under- 

 corrected for colour, not merely gain the best photographs, but did 

 the best work by lamphght." ..." Now, in view of the 

 irrationality of dispersion, absolute achromatism is impossible, and 

 in aiming to approach it as closely as may be, the corrections for 

 spherical aberration are inevitably sacrificed. It appears im- 

 portant that this fact should be more generally known." He then 

 proceeds to describe a simple test. It is gratifying to find my 

 views expressed in July so fully confirmed in November by our 

 American savant. 



There is one point upon which I desire to make an observation. 

 Dr. "Woodward says, in quoting a passage from my paper of Feb- 

 ruary, 1872, " It seems probable to me, therefore, that the distin- 

 guished makers last named have made no substantial progress since 

 1869 ; and this view is confirmed by Dr. Eoyston-Pigott, who 

 mentions (p. 66) that Powell and Lealand had placed at his dis- 



