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III. — EemarJcs on the Confirmation given hij Dr. Colonel 

 Woodivard to the " Colour Test." 



By Dr. Koyston-Pigott, F.E.S., &c., &c. 



I HAD the honour, on May 31, 1869, of transmitting to the Couucil 

 of the Eoyal Microscopical Society a paper printed seven months 

 afterwards, December, 1869, which contained the following state- 

 ment : — 



" I cannot here too strongly call attention to the beautiful 

 phenomena which I have always endeavoured to obtain as a fine 

 and reliable test of approaching aplanatism (freedom from spherical 

 aberration) and heralding a fine definition. 



" In examining striated bodies — 



" Longitudinal bands ghsten with a ruby tint upon a green or 

 yellowish ground ; " and describing the beads on the Podura scale, I 

 stated, p. 300— 



" The upper beads are best seen either green upon a pink ground 

 or pink upon a greenish ground," and p. 302 — 



" The most difficult definition is that of the substratum of beads 

 glimmering through the membrane nearest the light : on the other 

 side they are generally of a very bright yellow green colour, 

 contrasting prettily with the deep ruby colour of the upper beads." 

 —1869. 



It is gratifying to find the truth of this observation verified by 

 so distinguished and accurate a gentleman as Dr. Woodward, in his 

 communication for the November number of the ' Monthly Micro- 

 scopical Journal,' 1871, in which he alludes to the colour testing 

 in very decisive language. 



He says in substance, that purchasers of objectives, in general, 

 demand approximate achromatism above all else ; that in order to 

 obtain it, "the corrections for spherical aberration are inevitably 

 sacrificed ; " that in obtaining on a white screen by sunlight a picture 

 of Pleurosigma Formosum, it was found quite impossible to get a 

 distinct view of the beads unless the colour corrections were such 

 that they appeared, " brilliant red on a greenish ground, and that 

 when the object-glass is more nearly achromatic, not merely photo- 

 graphs are unsatisfactory, but with white light it wiU be found im- 

 possible to separate the beads distinctly on the screen." He then 

 mentions a variety of excellent glasses by difierent makers, which 

 all resolved the 19 th band of Nobert, but all of these had the same 

 colour corrections. 



The announcement which I had the boldness to make so early 

 as May, 1869, that in " the best glasses there is a certain residuary 

 aberration," raised a storm of opposition hardly yet subsided. I 



