AUGUST 19, I915| 
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 is 
taken of cnonymous communications. ] 
Colour Sensation. 
In Nature of July 15 there is given an abstract of 
a paper by Dr. F. W. Edridge-Green in which he 
gives reasons for supposing that the sensation pro- 
duced by spectral yellow is a simple sensation, and 
not a compound of red and green, as supposed by 
the Young-Helmholtz theory. In 1872 I read a paper 
on colour sensation before the Royal Scottish Society 
of Arts, in which practically all the experiments 
described by Dr. Edridge-Green are given, but the 
conclusion come to was that spectral yellow gave a 
compound sensation, because it could be altered by 
fatiguing the eye with either red or green. I have 
lately repeated these tests, and find my eyes still give 
the same reactions. To my eyes, the best yellow 
is just on the green side of the D lines in the spec- 
trum, but if the eye is fatigued with red light the 
yellow is changed to greenish-yellow, and if fatigued 
with green it is changed to orange, that is, the 
yellow changes in colour towards the unfatigued 
sensation. Testing by means of a sodium flame 
gives the same result. In making this test, the flame 
should not be used immediately after lighting it, as 
it then contains, in addition to the bright yellow 
lines, a continuous spectrum. The test should be 
made when the flame is nearly burnt out and the 
salt crackling. In that condition only the yellow 
lines are visible, and it shows the change better than 
in its first condition. To the eye fatigued with red 
it has a distinct greenish hue, while to the other eye 
fatigued with green it is reddish. The change is 
quite marked, and there is no hesitation as to the 
conclusion. 
With regard to the results given in the abstract 
of Dr. Edridge-Green’s paper: (1) Fatiguing the 
eyes with pure yellow spectral light could hardly be 
expected to affect the hue of the red, and could only 
slightly diminish its brightness, which will be diffi- 
cult to detect. (2) It is difficult or impossible to com- 
pare results when the eye in one case is flooded all 
over with a bright light, and the other in which it 
is fatigued with a narrow strip of weaker light 
which cannot be kept on the same part of the retina 
for any length of time. Results (3) and (4): Dr. 
Edridge-Green’s eyes and mine do not give the same 
results as stated at the beginning of this letter. 
It may be stated that the after-image seen on a 
white surface after looking at spectral yellow, and 
by the sodium flame, is the same as that given by a 
yellow compounded of red and green. Now if 
spectral yellow gave a simple sensation we would 
hardly expect this, as the after-image is a compound 
of all the unfatigued sensations, and we would expect 
spectral yellow if simple ought to give a nearly white 
after-image, and not a violet one. ; 
There is an old saying that ‘‘ Seeing’s believing.” 
In ordinary matters this may be so, but the belief is 
not necessarily true, and in questions of colour’ full 
of pitfalls. No two pairs of eyes see colours alike. 
This does not refer to colour-seeing and colour-blind 
eyes only, but there is reason to believe that all 
eyes differ more or less in their perception of colour, 
NO. 2390, VOL. 95] 
NATURE 

673 

and questions of this kind can only be settled by the 
decision of many eyes, and their decision only applies 
to themselves. Joun AITKEN. 
Ardenlea, Falkirk, August to. 

‘The History of Upper Assam, Upper Burmah, and 
North-Eastern Frontier.’’ 
I nave had the opportunity of looking through this 
recent publication, which was reviewed in NATURE, 
vol. cxiv., p. 481, December 31, 1914. It is most 
interesting reading to anyone, but particularly to 
those like myself who spent many years on the 
Eastern Frontier among the tribes mentioned. This 
book covers so large an area, goes back so far into 
the past history of Assam since the East India Com- 
pany were brought into relation with that country, 
and further back into a lost history of its ancient 
greatness, that 256 pages cannot do the subject 
justice. Very much which has been written of 
Assam, particularly in the Journal of the Asiatic 
Society of Bengal and other publications, is not re- 
ferred to at all, while certain parts .are very 
sketchy, even inaccurate. At the same time, very 
much that Colonel Shakespear has written is ex- 
tremely interesting, and his knowledge and experi- 
ence are very considerable, from having served so long 
in the country, and his conclusions sound. On p. 
108, treating of events in the Dafla country in 1874, 
when Colonel Stafford commanded a force which 
entered the country, we find: 
““The Daflas made no resistance, but paid up fines 
and returned the captives. Little or nothing was 
done by this large force in exploration or survey, 
and it returned to Assam amidst a clamour from 
Government over wasted money’? (the words in italics 
are mine. 1 would answer this statement by the 
following extracts from Reports, etc.). 
I. The Indian Museum, 1814-1914. Calcutta, 1914. 
“Bhutan is still an unknown country to naturalists, 
and its territory represents the most important gap in 
our geographical knowledge of the Himalayan fauna. 
East of Bhutan two expeditions of very different date 
are of zoological importance, namely, the Dafla ex- 
pedition of 1874-75 and the Abor expedition of 
1otI-12. On the first of these, Godwin-Austen, then 
a Major in the Bengal Staff Corps attached to the 
Survey of India, himself made collections of great 
value, and also encouraged his subordinate officers 
to do the like.” 
Il. As to the survey work. 

In a Memoir on the 
Indian Surveys, by Clements R. Markham, C.B., 
F.R.S. India Office Publication. 1878, p. 173- 
“In 1875 Major Godwin-Austen accompanied the 
Duffla military expedition against the tribes on the 
northern frontier of Assam. Narainpur, on the Dik- 
rung Nullah, was reached on December 2, 1874, and 
from a base on the banks of the Bramaputra a 
short series of triangles was extended northward into 
the Duffla hills. Owing to the brief period during 
which the military were in the country, the survey 
party were unable to remain beyond two and a half 
months. The out-turn of work amounted to 1705 
square miles of entirely new topography, on the scales 
of two and four of the season’s work in the Duffla 
hills, has been compiled by Major Godwin-Austen, 
and is a valuable addition to our geographical know- 
ledge of the region beyond the northern frontier of 
Assam. Lieut. Harman, R.E., rendered assistance 
by surveying the course of the Ranga river; and Mr. 
Lister, of the Royal Botanical Gardens, Calcutta, 
was assiduous in making a collection of plants, seeds, 
