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X LV. On a new Standard of Light, By Louis Schwendler*. 

 [Plate XI. figs. 7-10.] 



NO exact measurement of any quantity, even with the 

 most accurate and sensitive test-methods available, can 

 reasonably be expected unless the standard by which the un- 

 known quantity is to be gauged is perfectly constant in itself ; 

 or, if nature does not permit of such a desirable state of things, 

 the causes to which the variation of the standard are due 

 should be known, and in addition also their quantitative effect 

 on the standard, in order to be able to introduce a correction 

 whenever accuracy of measurement should permit and circum- 

 stances necessitate it. 



This requirement for a standard necessarily entails on the 

 one hand a knowledge of the relations which exist between 

 the standard and the causes of its variation, and on the other 

 hand the possibility of an accurate and independent measure- 

 ment of these causes. 



Further, having no constant standard, it is impossible to 

 produce two quantities of the same kind bearing a fixed 

 and known ratio to each other ; consequently no idea can be 

 formed of the accuracy of the test-method adopted ; and if such 

 is impossible, we are also unable to improve the test-method in 

 itself, i. e. with respect both to accuracy and sensitiveness. 



The inconstancy of a standard acts therefore perniciously 

 in two directions : it prevents us from being able to execute 

 accurate measurements even with the most accurate and sen- 

 sitive test-methods, supposing such are available, and, further, 

 leaves us in the deplorable condition of not being able to 

 improve the test-method although we may be convinced that 

 the method of testing requires such improvement. 



It may be safely asserted that in any of the branches of the 

 physical sciences where constant standards do not exist the 

 progress in accurate knowledge of nature must be slow, if not 

 impossible. 



This train of thought will, I think, invariably beset the 

 physicist who endeavours to make photometric measurements. 



Recent experiments on the value of the electric light as 

 compared with the ordinary means of illumination | called my 

 attention forcibly to this point. 



* From the Journal of the Asiatic Society of Bengal, vol. xlviii. part 

 ii. 1879. Communicated by the Author. 



t These experiments I had to institute on behalf of the Board of Di- 

 rectors of the East-Indian Railway Company, under orders of the Secre- 

 tary of State for India to inquire into the feasibility and practicability of 

 lighting up Indian Kailway-stations by the electric light. 



