318 
NATURE 
[FEBRUARY 4, 1897 
practical calculations go, Prof. Perry, in getting out in foot- 
pounds the energy of rotation of a fly-wheel, or in making a 
conversion into horse-power from C.G.S. units, merely does 
his division by g (arithmetically or symbolically) at an early 
stage, where a ‘* poundalist ’ would probably divide at the end. 
But with the elementary student it is different ; a misconception 
at the start may stick to him for years unnoticed. Perhaps an occa- 
sional bath among all sorts of difficulties in which he is told to 
sink or swim, such as Prof. Perry advocates, may have a stimulating 
effect. But let us consider. Prof. Perry’s main complaint was 
that so many are overwhelmed by the poundal. (Oddly enough, 
he does not seem to mind the dyne, which is on all-fours with it ; 
in fact he has nothing to say against the work of the B.A. Units 
Committee, beyond a grumble at the fact that they acquiesced in 
the formula 477” as representing the surface of a sphere.) Now, 
for instance, will a student not be puzzled as to the reason why 
this new system should bear to the English standards a relation 
so utterly unlike that which the C.G.S. system bears to the 
French standards? And it seems to me that there must be 
students who might manage to keep afloat in presence of the 
poundal to whom Prof. Perry’s definition, that ‘“the unit force 
is that which would give to a body of 32°18 times the inertia of 
the standard object kept in London an acceleration of 1 foot 
per second per second”—invented out of pure kindness for 
them—may prove a very millstone round their necks. And 
they will be no better off if he raises the talismanic number to 
32°19, as apparently he would now do. 
May I conclude by suggesting to Prof. Perry, by way of a 
small return for the many useful things which I have learnt from 
him during the past twenty years and more, that an appreciative 
toleration of rule-of-thumb methods is one thing, but it is 
another thing to glorify and perpetuate them. 
Ashurst Wood, January 12. M. J. JACKSON. 
Durham Science Degrees. 
I AM surprised to see the letter of your anonymous corre- 
spondent ‘‘ X.” in your issue of January 28. 
All the six gentlemen referred to, hold important positions on 
the teaching staff of this college. 
They were recommended by the Council of the college to the 
Senate of the University as deserving such a degree as would 
make them members of the Convocation of the University. 
‘“*X.” insinuates that these gentlemen had no qualification for 
this honour. 
The truth is, that one of them is M.Sc. of Victoria Univer- 
sity, three of them are B.Sc. of Scotch Universities ; the remain- 
ing two are the Senior Lecturers in their respective departments, 
where they have taught graduates of the University for twelve 
and thirteen years respectively. HENRY PALIN GURNEY, 
The Durham College of Science, Newcastle-upon-Tyne, 
January 29. 
PHOTOGRAPHY IN COLOURS. 
HE announcement has been made during the past 
week of the discovery of a true process of photo- 
graphy in colours. It is too early now to discuss the 
matter in its many interesting bearings, since the process 
so far remains more or less secret, but the following com- 
munications will indicate how the question at present 
stands. 
The first is a communigué received from Sir H. 
Trueman Wood, the Secretary of the Society of Arts. 
I am anxious to make, through the medium of the 
Society of Arts /ourna/, at all events a preliminary 
announcement of a very remarkable process for produc- 
ing photographs in colours which was brought to my 
notice the other day. To say that it enables photographs 
to be produced in natural colours would not, perhaps, 
be precisely true, since colouring media are employed ; 
but the result of the process is a photograph in colours 
of nature—a faithful reproduction in colour of the object 
photographed—and so, for all practical purposes, it may 
be said that the long-sought object of photographic 
NO. 1423, VOL. 55] 
research, photography in colour,, has actually been 
obtained. 
The inventor is M. Villedieu-Chassagne, of Paris, who 
has developed a process originally suggested by Dr. 
Adrian Dansac, and the following is his method :—(It 
must be premised that he keeps secret, at all events for 
the present, the nature of the four solutions he employs.) 
A negative is taken on a gelatine plate prepared by 
treatment with one of his solutions. This is developed 
and fixed in the ordinary manner. It shows no trace of 
colour. From it a print is taken on glass or paper, the 
plate or paper being specially prepared by treatment 
with the same solution. The transparency or the paper 
print in no way differs, to all appearance, from an ordinary 
positive, and shows no trace of colour by transmitted or 
by reflected light. It is then washed over successively 
with three coloured solutions, blue, green, and red, and it 
takes up the appropriate colours in appropriate parts, 
these three colours giving, by their various combinations, 
all varieties of hue. How it is that this power of selective 
absorption is given to the components of the photographic 
image (principally, of course, metallic silver) is, it appears 
to me, the interesting question connected with the pro- 
cess. The action is certainly previously unknown, and it 
will, as certainly, repay scientific investigation. 
As I declined to be convinced by mere inspection of 
the finished results, M. Chassagne was good enough to 
demonstrate the whole process for my benefit, and by 
the kindness of Prof. Thomson, of King’s College, the 
demonstration was allowed to take place in the laboratory 
of King’s College on two mornings last week. Prof. 
Thomson and Mr. Herbert Jackson, of King’s College, 
were present on both occasions, and Captain Abney on 
the second. I must not speak for those gentlemen, but 
I believe they were as much impressed as I was myself 
by the remarkable nature of the process and its results. 
That such results should be obtained by sucha process 
seemed ¢ frovz in the highest degree improbable, but 
obtained they certainly were. 
The photographs taken by ourselves were poor, the 
light (on the morning of Wednesday, 2oth) being ex- 
trenrely bad. Nevertheless, the positives (made by one 
of ourselves on the following day) showed with perfect 
distinctness, when treated as above described, the colours 
of a bunch of flowers I had bought at Covent Garden, 
on my way to King’s College, and of various other test 
objects. 
Our own experiments were confined to gelatine films, 
but M. Chassagne treated with complete success some 
paper positives he had brought from Paris. These looked 
like ordinary silver prints toned with gold, but I omitted 
to ask about the toning. 
Further experiments and independent investigation 
(for which M. Chassagne has kindly promised me the 
materials) will, no doubt, throw further light on the nature 
of the process ; but I cannot believe that any investiga- 
tion will throw doubt on its genuine character, for it was 
carried out under test conditions last week, the sole re- 
servation being the nature of the materials employed. 
I hope that a fuller account of the method may shortly 
be presented to the Society in the form of a paper; but 
in the meantime it appeared to me that members of the 
Society would be interested by having placed before 
them the first information about so remarkable and 
promising an invention. H. TRUEMAN Woop. 
Captain Abney, who was present at the experiments 
referred to above, writes as follows :—- 
The process of colour photography, which I had the 
pleasure of seeing demonstrated at King’s College 
some ten days ago, through the kindness of Sir H. T. 
Wood, is a very remarkable one. I went asa sceptic. 
