1879.] L. Schwendler—On a new Standard of Light. 83 
VIII.— On a new Standard of Light. By Lovis SCHWENDLER. 
(With Plate VIII.) 
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 unknown quantity is to be gauged is perfectly con- 
stant 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 measure- 
ment should permit and circumstances 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 measurement 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, ¢. ¢., with respect both to accuracy and sensitiveness. 
The inconstancy of a standard acts, therefore, perniciously in two direc- 
tions: it prevents us from being able to execute accurate measurements 
even with the most accurate and sensitive test-methods, supposing such are 
available ; and further leaves us in that 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. 
* These experiments I had to institute on behalf of the Board of Directors of the 
East Indian Railway Company, under orders of the Secretary of State for India, to 
enquire into the feasibility and practicability of lighting up Indian Railway Stations 
by the Electric Light. 
