536 



C. PIAZZI SMYTH ON THE 



To the above numerical particulars, the following details of focus, taken on 

 b June 21 may be usefully added. 



The Collimator being kept at a fixed focus, the telescope's focus tried 

 on the Solar lines of the Grating's spectrum with an eye-piece magnifying 

 67 times, was found to be at the following successive distances in inches 

 from its objective : — 



At Solar Lines. 



1st Order of Spectrum. 



2nd Order of Spectrum. 



3rd Order of Spectrum. 



Great A 



(68-55) ? 



Mixture of Spectra. 



Mixture of Spectra, 



Little " a " 



68-43 





shown by red and 



B 



68-39 



B = 68-57 



blue lines inter- 



C 



68-36 



C = 68-50 



mingled. 



D 



68-31 



D = 68-43 



D = 6846 



E 



68-34 



E = 6843 



E = 6846 



F 



68-39 



F = 68-46 



F = 68-52 



F to G 



68-47 



F to G = 68-55 



Mixture of Spectra 



G 



68-59 



G = 68-70 



again, red and blue 



H 



(68-72) ? 



Mixture of Spectra. 



lines intermingled. 



These numbers show that Messrs Cooke's form of achromaticity of 

 objectives, gives a more nearly uniform focus throughout the spectrum than is 

 generally met with. Yet that very excellence rather conduces sometimes to 

 the mistaking of intermingled lines of two adjacent orders of the Grating's 

 spectra. 



Such intermingling was probably less on this occasion than it often is, by 

 reason of the employment of a large anterior prism, which never allowed 

 a white image of the Sun to fall on the Slit, from the Heliostat's condenser lens 

 (6 inches in diameter and 90 inches focus) ; but spread it out sideways as a 

 short spectrum-coloured smear. Of very "impure" spectrum-character no 

 doubt, yet enabling the greater mass of each individual colour to be thrown 

 separately on the slit at will. 



The Prism was of moderately good Flint glass, of 38° Refracting Angle, and 

 with the faces enclosing that angle, measuring 5 by 5 inches. It was inserted 

 transversely into the cone of Solar rays coming from the Heliostat lens, 

 65 inches before they arrived at a focus ; and the large spectroscope table, 

 carrying collimator, grating and telescope on its surface, rotated horizontally 

 round a vertical axis under the centre of the prism ; and could afterwards be 

 adjusted slowly or definitely fixed by a grand screw motion in tangential 

 direction at the outer end of the whole and on the floor level, — when the colli- 

 mator had been placed by trial in the line of minimum deviation for the prism 

 at the part of the Spectrum under observation with the Grating. 



In the third Winchester Spectrum, more use was made of coloured glasses 



