90 Dr. E. Rose on the Doctrine 



that their spectrum is not shortened, that the red end takes 

 up so much less room than the blue, that on this account a 

 shortening at the red end might thus perhaps escape notice. 



It can only be said that when there are more than two spaces 

 the spectrum is certainly shortened, and not conversely. That 

 this is the case I have since then convinced myself. 



It was now the question to be able to ascertain, by means of 

 a simple instrument which could be carried in the waistcoat- 

 pocket, the existence of a shortening of the spectrum — that is, to 

 show colour-blindness in the case of the colour-diseased. It 

 was also of interest to ascertain whether with two intervals a short- 

 ening of the red can occur — whether, in fact, there are colour- 

 diseased persons without colour-blindness, and how far the spec- 

 trum in the case of those with six intervals may be shortened. 



At present, when the spectrum-apparatus of Bunsen and 

 Kirchhoff is everywhere met with, it is easy to be convinced 

 on the subject. In order to have a sharply terminated spectrum 

 and to be able to deaden the bright yellow, one of the large ap- 

 paratus with rotating ocular telescope is necessary. All false 

 light must moreover be carefully excluded by black cloths. A 

 spectrum is thus obtained which even in artificial light is tole- 

 rably sharply defined, is subjective, and is hence not liable to 

 fluorescence, and, by using simple chemically coloured flames, can 

 always be obtained of the same length. 



As a scale, the reflected image of the photographed scales can 

 be used instead of the Fraunhofer's lines, or, better still, the 

 natural scale as offered by the inspection of various metallic 

 spectra. In this way the frequent imperfection of the red 

 of the solar spectrum is avoided, which was probably formerly 

 often the reason why defects there were overlooked. Looking at 

 Bunsen's plate 5, nothing more convenient can be imagined for 

 deciding the question whether there has been a shortening than 

 the preparation of a potassium spectrum. 



A preliminary investigation, for which Dr. Alexander Mits- 

 cherlich, by means of his apparatus, kindly prepared the neces- 

 sary constant spectra, confirms sufficiently what I supposed and 

 stated years ago. 



After the sodium-line D had been fixed on 12 (as in Plates I. 

 and II., vol. xxviii. of this Magaziue), a potassium spectrum 

 was produced by the vapour of cyanide of potassium heated 

 in hydrogen ; by a platinum wick, a chloride-of-strontium spec- 

 trum ; by a cigar-stump, a lithium spectrum ; and finally, by a 

 simple lamp, a coal-gas spectrum*. The results were as follows : — 



* Compare " Contributions to Spectrum-Analysis," by A. Mitscherlich, 

 vol. cxvi. p. 499 [abstracted in Phil. Mag. vol. xxiv. p. 519]; and "On 

 Spectra of Compounds and of Simple Bodies," by the same, vol. cxxi. p. 459 

 of Poggendorff's Annalen [translated in Phil. Mag. vol. xxviii. p. 169]. 



