Sir Graves C. Haughton in Reply to Mr. Joule. 435 
The picture is beautifully distinct, but of very short duration, 
and is not succeeded by any accidental or complementary im- 
pression, owing to the carpet being faintly illuminated. The 
pink ground is obviously a combination of the original green 
ground with its very faint complementary red, while the dark 
red pattern has had its redness deepened by its own com- 
plementary green. 
When this experiment is well made, which can be done 
only when the eye is very sensible to luminous impressions, 
the observer feels as if he possessed two eyelids which are 
closed in succession, with an interval of one third of a second, 
the jirst or the real eyelid shutting out the original object, 
and the second or the imaginary eyelid shutting out the pic- 
ture composed of the prolonged direct impression, and its co- 
existing complementary impression. If the eyes are kept very 
steady with the intersection of their axes fixed upon the 
centre, or any other definite point, of the red pattern, the 
eyelids may be shut and opened any number of times in suc- 
cession without injuring the brightness and distinctness of the 
combined impression. ‘The picture indeed becomes more and 
more distinct, and if a considerable degree of illumination is 
employed, the single complementary impression may be made 
visible after the component one has vanished, or rather after 
the prolonged direct impression has disappeared from the com- 
pound one. , 
St. Leonard’s College, St. Andrew’s, 
April 29, 1843. 
LXXII. Remarks on Mr. Joule’s Explanation of Experi- 
ments on the Galvanometer. By Sir Graves C. Haucuron, 
ERS. 
To the Editors of the Philosophical Magazine and Journal. 
GENTLEMEN, 
] HAD the pleasure of reading Mr. Joule’s explanation of 
my experiments on the galvanometer which are printed 
in your Journal for March; and while I perfectly agree with 
him that the movements observed in the needles were due to 
repulsion, I think that the means I took to ascertain that fact, 
and which were known to me previous to reading his letter, 
may be more convincing than the experiment which he gives 
in illustration, though they are really founded on the same 
principle. 
Observing that the needles en instantly attracted by the 
2G 2 
