JULY 19, 1923 PROCEEDINGS: PHILOSOPHICAL SOCIETY 299 
tidal wave at Hilo, Hawaii, are of special interest. The automatic tide- 
guage records at twenty stations are now regularly examined for abnormalities 
due to earthquakes. 
Possible relations between the theory of isostasy and earthquakes are 
pointed out. Isostasy indicates that earthquakes occur above the depth 
of compensation. 
Dr. J. E. Ives presented a paper on The nature of the illumination used by 
engravers of steel plates. ‘The paper was discussed by Mr. HawkKEesworrtu. 
Author’s abstract: The engraver on steel works on highly polished steel 
plates which are used in the printing of pictures, postage and revenue stamps, 
paper money, certificates of stock and bonds. Since the lines cut into the 
steel are visible only by the shadows cast by the edges of their grooves or 
depressions, or because their surfaces are inclined at such angles that they 
are in shadow or do not reflect light falling upon them, the best results are 
obtained by using an illumination which is diffuse with a predominating 
direction downwards and towards the worker, and which produces a uniform, 
or nearly uniform, brightness of the whole surface of the plate. This is, in 
practice, obtained by using a screen of translucent material such as tracing 
cloth, tracing paper, or tissue paper, placed between the source of the illumi- 
nation, natural or artificial, and the plate, so that the image of the tracing 
cloth is reflected in the plate. The screen is usually about 30 inches square 
and inclined at an angle of about 45° to the plate. The reflected image of the 
illuminated tracing cloth makes the polished surface of the steel look like a 
sheet of white paper. On such a surface the lines cut into the steel can be 
clearly seen. If such a screen is not used there will, in general, be bright and 
dark areas on it, and where the surface is dark the lines cut into the surface 
cannot be seen. Also the contrast between the bright and dark areas on the 
plate produces more or less glare. 
Measurements were made of the illumination on the plates, of the bright- 
ness of the screens, and of the brightness of the surface of the plates. Study 
of the results obtained leads to the conclusion that the brightness of the 
surface of the screen should be as uniform as possible. For natural illumina- 
tion an uninterrupted north sky light is the best, and is much better than 
light from south windows, since the latter is very variable, varying from 
bright sunshine to cloudiness. If the screen is illuminated artificially by 
one or more electric lamps, the light from the lamps should be so arranged 
that it is spread uniformly over the screen. It was found that a non-uniform 
illumination of the screen produced discomfort in the eyes of the workers. 
Artificial illumination, besides being uniformly distributed over the screen, 
should also be of sufficient intensity. 
The results recorded in this paper were obtained in connection with 
investigations on this subject made recently by the office of Industrial 
Hygiene and Sanitation of the U. 8. Public Health Service. 
884TH MEETING 
The 884th meeting was held jointly with the Washington Academy of 
Sciences and the Chemical Society of Washington in the Auditorium of the 
Interior Building at 8:30 p.m. on Tuesday, April 17, 1923. 
Papers were presented by Dr. F. G. Donnan, Professor of Chemistry, 
University College, London, and by Dr. Jamus C. Irving, Principal and 
Vice-Chancellor, University of St. Andrews, on their own recent researches 
in chemistry. 
