432 



Dr. Royston-Pigott on 



[June 19, 



The delicate measurement of the primary black ring and disk was 

 verified by a recording eyepiece micrometer. With this and the objec- 

 tive used it was found that one thousandth of an inch on the stage 

 measured 1138 divisions, i. e. eleven turns of the divided head and 

 fVu" °^ a t urn « O ne division therefore represented 



°nf8— i 138 ooo ~ rune ten-milhonths nearly. 



On estimating the breadth of the primary jet-black ring, and using a 

 ruled glass micrometer, as I could detect no difference in the breadths 

 of each ring, I felt justified in dividing the total diameter by the 

 number of rings in order to obtain the breadth of one, which gave j-eisT • 

 A much deeper point in the axis showed a very deep blue central haze, 

 paling outwardly, and then melting into a final red fringe. 



The precision of the mechanical construction of this fine objective was 

 thus revealed by the use of a simple convex lens of crown glass. Any 

 deviation from accuracy was at once detected by the converging pencil 

 of the plano-convex lens, consisting of shells of rays of various re- 

 frangibilities, having their several foci arranged along the axis. As 

 this axis was necessarily single and unique, the interference phenomena, 

 especially the sharpness and intensity of the jet-black rings, could only 

 be so superbly exhibited by the best glasses. Inferior glasses blurred 

 them and dulled the rich beauty of the colours. 



Contrasting with this the performance of a variety of glasses, both 

 English and foreign, very peculiar appearances arose which doubtless 

 indicated grave errors of construction. I will venture briefly to mention 

 some of these : — 



(I.) A variety of spurious disks oddly arranged were displayed. 



(II.) The beauty of the rings was entirely marred ; and 



(III.) Very few rings could be developed, and sometimes no black rings 

 whatever (PL II. fig. 7, and PL III. fig. 14). 



(IV.) Notched, grained, and spotted ; the rings were sometimes irregular 

 in shape (fig. 12). 



(V.) A multitude of fine black excentric rings, evidently arising 

 from different centres, were seen upon a leaden-grey field surrounding 

 the central disk, the confusion of the rings causing a bad achromatism. 



(VI.) An " engine-turned pattern " was not unfrequent, degenerating 

 into a peculiar grained and mottled appearance. 



(VII.) A majority of the glasses were overcorrected spherically. 



(VIII.) Achromatism and aplanatism in our best adjustable glasses 

 were found to be altogether incongruous. 



As this result was alluded to at page 428, I proceed to relate the cir- 

 cumstances of the observation. I constantly found in pursuing these 

 researches that either achromatism was sacrificed to aplanatism, or that 

 the attainment of achromatism destroyed the brightness and truth of 

 aplanatism. I may relate the following experience. 



Whenever in watching the heliostat the sun was clouded over, as 



