558 Intelligence and Miscellaneous Articles. 



perpetual standards of comparison had been introduced; and although 

 a great step was taken by the suggestion to use a universally recog- 

 nized scale of colours as a point of reference (for which aim was 

 given a chromic plate in the book), coupled with the mentioned use 

 by Mr. Huggins of chemical solutions as such standards, many hin- 

 drances were left remaining ; and in the great loss by Admiral 

 Smyth's death shortly afterwards was probably included much fur- 

 ther progress towards their removal. 



The only instrumental means described by him, the photometrical 

 measurement of the spectrum of the star so as to determine the 

 lucidity of its different sections, is objectionable, as well by reason 

 of its exceeding dependence on the occurrence of opportunities of 

 weather not only "fine," but "superlatively fine," as by reason of 

 the great and numerous difficulties which render the application of 

 it almost impracticable. The object of the author is to describe an 

 apparatus for the purpose of determining star-colours, by which the 

 tints of the fixed stars may be exactly recorded relatively to standards 

 easily reproducible by any observer, with any kind of telescope, any 

 number of years hence, and that by a contrivance the manipulation 

 and reading of which is as easy as the plans now usually adopted for 

 photometric estimations. But he first recapitulates the causes of 

 error which particularly belong to this kind of research. These are : — 



1st. Personal Equation, including therein three heads, which, 

 although properly so described as belonging entirely to the persona- 

 lity of the observer, are actually distinct, viz.: — (A) That insensibility 

 of the eye to the varieties of colour which in its most extreme form 

 is colour-blindness. (B) Inability of the memory to retain exactly 

 the impression produced by a certain tint, so as to be capable of re- 

 producing and identifying it at a subsequent period. (C) Personal 

 difference in the habit of describing the impression of a particular 

 colour. 



2nd. Atmospheric equation. 



3rd. Instrumental Equation. Good achromatic refracting tele- 

 scopes are open to little imputation of deceit as regards the exhibi- 

 tion of the colours of celestial objects ; but the case is far otherwise 

 with reflectors. The prevalence of excessive redness among Sir W. 

 Herschel's chronicles of sidereal chromatics has long given rise to the 

 opinion that the speculum-metal misled him in this respect ; and in 

 the same way silvered glass mirrors are not (without due correction) 

 reliable in any case where the colour of an object is to be accurately 

 depicted. 



4th. Standard of comparison. The requisites in such are that it 

 shall afford the exact shade of colour of the star in connexion with 

 which it is to be used, so that such tint shall be easily reproducible 

 with precision by any observer at a future time from the information 

 transferred by the ordinary use of language, and that it shall be suit- 

 able for comparison with telescopic images. A painted scale like 

 that given in ' Sidereal Chromatics,' by Admiral Smyth, is, on ac- 

 count of its opacity of colour, objectionable, and can scarcely claim 

 to be considered sufficiently reproducible. Precious stones, though 



