97 
1879.] L. Schwendler— 'Electric Light Measurements. 
(Illustration of the Centimetre-Gramme-Second (C.G.S.) system of 
units, by J. D. Everett.) 
In Appendix I.* attached to the Eeport, all the required data are given 
for easy reference. 
Means oe Measurement : 
The Tangent Galvanometer which is described in Appendix (II) of 
the Eeport. 
By means of this instrument, constructed in special manner to suit 
the particular purpose, all the currents between 1'6 to 38 wehers could 
be measured very accurately. 
The currents, in absolute measure, may be taken as correct within 3 
per cent, at least. The Astronomer Eoyal most kindly gave me the value 
of the horizontal component of the earth’s magnetism at Greenwich. 
The Photometer described in Appendix III. The comparison of two 
lights by means of this instrument rests on the following principle :— 
Two surfaces, equal in size and nature, are placed side by side, quite 
close together, and in the same plane. 
One surface is illuminated by one light, the other surface by the 
other light. 
This is best done by using two equal flint-glass prisms. 
The surfaces towards the eye are covered with tracing paper, the top 
half of each paper being divided by fine pencil lines into small squares. 
The equalization of the intensities of the two lights under comparison 
is done by keeping the distance between the two lights constant, and 
moving the two illuminated surfaces within that, constant distance. 
This Photometer is particularly well 'adapted for comparing the in¬ 
tensities of two differently coloured lights. 
An experienced eye can easily distinguish the relative distinction 
between the pencil line and the back ground of each surface, and if this 
relative distinction is the same in both surfaces, the two surfaces have equal 
intensity, no matter what the colour of each light may he. However, as a rule, 
the comparison of any two lights was made by observing the two surfaces 
through a red glass. This glass, after trying a great variety of glasses, 
was found to have the convenient quality of making the colours of the 
electric light and of the standard light equal. 
It was ascertained that this red glass absorbs all the violet, blue, and 
green rays, letting through only some yellow, more orange and all the 
red. 
* All the Appendices referred to in the following belong' to the actual Report, of 
which the present paper is a Precis only. 
