AUGUST 22, I912] 
NATURE 
Oi 
By G. E. Brown and 
(London: Henry Green- 
Price rs. net. 
Photographic Copyright. 
A. Mackie. Pp. 89. 
wood and Co., 1912.) 
Acts of Parliament are not always intelligible 
to even the legal mind, and the ordinary person, 
whose pmwvileges and duties are therein defined, is 
often much troubled to know what the law really 
is. Therefore, everyone who makes or has to do 
with photographs is much indebted to the authors 
for clearly stating how the matter of copyright 
stands. They first give on a small page the short- 
est possible statement of the new Copyright Act, 
pointing out those parts wherein the new Act 
differs from the one that preceded it. They then 
take up just those points upon which anyone read- 
ing the Act would like a little more information, 
and make them clear, often quoting judicial 
decisions where the interpretation of the Act would 
otherwise be doubtful. The volume closes with 
a tabular statement of the copyright laws of other 
countries, the text of the 1911 Act, the 1862 Act 
so far as it is not repealed, an excellent index, and 
a list of the most important copyright cases that 
have been decided in the Courts. 
LETTERS TO THE EDITOR. 
{The Editor does not hold himself responsible for 
opinions expressed by his correspondents. Neither 
can he undertake to return, or to correspond with 
the writers of, rejected manuscripts intended for 
this or any other part of Nature. No notice ts 
taken of anonymous communications.] 
Artificial Daylight. 
May I supplement the interesting article on the 
above subject which appeared in Nature of August 15 
by a short note on some work carried out about 
twelve years ago by Mr. Arthur Dufton and myself? 
The work had for its primary object the removal of 
the great difficulty experienced by dyers and other 
workers in colour, under the extremely variable con- 
ditions of illumination which naturally prevail. These | 
conditions are such that during the winter months a 
few hours only per day are available for the accurate 
matching of colours, and even in the most favour- 
able circumstances the colour-quality of daylight is 
continually varying. A standard light, which would 
render dyers and others independent of atmospheric 
conditions, was therefore much needed, and _ the 
*Dalite "’ lamp. which was the ultimate outcome of 
our work, solved the problem in such a satisfactory 
way that many hundreds are in use in dye-houses, 
colour-printing works, schools of art, drapery estab- 
lishments, &c., both in this country and abroad. 
For accurate colour work it is, of course, not 
necessarily sufficient to have available a ‘‘ white” 
light. In addition to being devoid of colour, the light 
must obviously contain all the vibrations of the visible 
spectrum in properly balanced proportions. The 
correction of a single radiant which does not yield 
a complete and continuous spectrum is impossible. 
Our work, therefore, consisted in the examination 
of all available illuminants, the selection of the most 
suitable, and the elimination from the selected light 
of those rays whicn were found to be present in 
excess. 
NO. 2234, VoL. 89] 
Some account of the work has been given in papers 
| read before the British Association (Bradford meeting, 
1900), the Society of Chemical and the 
Society of Dyers and Colourists. 
We found that an are lamp of the enclosed type, 
burning pure carbons, gave the nearest approximation 
to the light favoured by colourists, i.e. that diffused 
from a cloudy north sky. An enclosed arc lamp 
yields light from two sources—the glowing carbons 
Industry, 
| and the are itself—and by adjusting the length of 
arc, diameter of carbons, &c., and thoroughly mixing 
by diffusion the light from these, we arrived at our 
raw" light. 
This was found to contain an excess of red and 
violet rays, and a long investigation was then under- 
taken to find the best means of cutting out this 
excess. Reflection of the light from white or tinted 
surfaces was a practical failure on account of the 
enormous loss in intensity, and eventually direct 
absorption by suitably tinted screens was adopted. 
The whole range of coal-tar green and blue colouring 
matters were examined in the form of dyed gelatine 
films, and the curious fact emerged that with one 
exception—naphthol green—all were fairly transparent 
to red light, and therefore unsuitable for the purpose 
of absorbing the red, though the absorption of the 
excess of violet presented no difficulties. 
With a suitably adjusted lamp, a solution of sul- 
phate of copper was found to give the necessary 
| absorption in the red, and after numberless expensive 
failures, a suitable blue copper glass was produced. 
In its final form the ‘‘Dalite’’? lamp consists of a 
carefully adjusted enclosed arc lamp surmounted by a 
lantern fitted with white diffusion and blue absorption 
glasses. Provision is not usually made for the absorp- 
tion of the excess of violet since this is not found to 
interfere with the accurate matching of hues,’ but 
by the introduction of a third glass this is readily 
provided for. 
The Moore light referred to by your contributor, in 
which the gas in a partially evacuated CO, tube is 
rendered incandescent by an alternating current, emits 
light containing an excess of green rays, which 
renders it inaccurate for many hues, e.g. pale pinks 
and blues, but otherwise it is a most suitable illu- 
minant for colour work. Watter M. GARDNER. 
Technical College, Bradford, August 16. 
Experimental Illustration of the Reversal of Bright 
Line Spectra. 
Tue following way of showing the reversal of the 
bright line spectrum of metals may be of some 
interest to lecturers. I have not, so far, seen the 
method described. 
Having scraped a hollow in the lower carbon of an 
are lantern (which should be non-automatic), fuse a 
little iron wire (for example) in the flame, keeping 
the carbons as far apart as possible. If the usual 
| spectroscopic arrangement is placed in front of the 
lantern a bright line spectrum of course appears on 
the screen or in the field of the telescope. 
Now by the hand adjustment reduce the distance 
between the carbons until the point of the upper 
carbon is practically within the crater. Suddenly the 
bright lines on the screen “reverse,’’ becoming 
bright once more as the poles are again separated. 
The production of reversal is evidently due to an 
envelope of relatively cool gases round a small arc, 
the envelope consisting of the outside layers of the 
gases in the original and much larger arc. 
1 The reasons for this have been thoroughly werked ou’, but are imma- 
terial to the present purpose. 
