Lord Rayleigh's Investigations in Optics, 403 



P. L. S. (A) = 0402 ohm at 18°-9 C. 



= 0*964 „ at the temperature of the standard 



where the light was produced, or increase =9*45. 



I regret that I have not been able to calculate from the 

 above results the temperature of the heated platinum, since I 

 could not procure in time a copy of Dr. William Siemens's 

 Bakerian Lecture (1871), which at present, to my knowledge, 

 is the only source whence the increase of resistance of platinum 

 at high temperatures can be found. 



To sum up, the advantages of the new standard of light are: — 

 The light is perfectly constant if the current be kept constant ; 

 it allows a correction to be made for the variation of the current 

 if this variation is known ; it can be reproduced accurately 

 everywhere if ordinary precautions be taken to secure pure 

 platinum*; its magnitude can be altered to any extent to suit 

 certain practical purposes by simply varying the elements of 

 weight, shape, and size of the platinum, or the strength of the 

 current passing through it ; it does not alter of itself either in 

 intensity, size, or position, and therefore by it most accurate 

 photometric measurements can be executed ; the standard can 

 be easily made to fit into any adopted system of absolute units. 

 Hence the new standard fulfils all the recognized conditions 

 of a perfect and rational standard ; and therefore it would be 

 advisable to adopt it in future as the practical standard for 

 light-measurement. There would be no practical difficulties 

 met with in the introduction of the new standard for technical 

 purposes. 



XLYI. Investigations in Ojrtics, with special reference to the 

 Spectroscope. By Lokd Eayleigh, F.R.S. 



[Continued from p. 274.] 



§ 4. Influence of Aberration. 



IN the investigations of § 2 the wave-surface was considered 

 to be plane, or (after passing through a condensing lens) 

 spherical. As all optical instruments are liable to aberration, it is 

 important to inquire what effects are produced thereby upon the 



* The conductivity of any metal is much lowered by slight impurities ; 

 and platinum does not form an exception ; hence great care must be exer- 

 cised in the selection of platinum for the light-standard. Dr. William 

 Siemens, in his Bakerian Lecture says : — " The abnormal resistance of 

 some platinum is due chiefly to the admixture of iridium or other metals 

 of the same group ; and it appears that platinum prepared by the old 

 welding process is purer, and therefore better suited for electrical purposes, 

 than the metal consolidated by fusion in a Deville furnace." 



