1879.] H. F. Blanford— -M. Janssen's Photographs of the Sun. jgg 
not less than nine diameters (81 times in superficial extent) for the purpose 
of convenient study. The specimen exhibited is one of these enlarged pho¬ 
tographs. 
The original pictures having represented the sun’s disk with a diame¬ 
ter of 30-5 centimetres, the enlarged photograph represented the central por¬ 
tion of a disk which, if complete, would be not less than 2745 metres 
or about 9 feet in diameter.* It showed in great perfection, the granular 
structure of the surface and that differentiation of parts which M. Janssen 
terms the ‘ risemphotospUrigue.' In certain areas, all the granules are 
more or less confused and drawn out as if swept along in a gaseous current • 
while, in the intervening tracts, they are distinct and rounded in form, pre¬ 
senting a series of brilliant dots surrounded by more shaded portions. M 
Janssen is now engaged on the study of the movements thus brought to 
light, and this may be done with comparative ease with photographs, which 
afford an exact representation of the solar surface taken at intervals of 3 or 4 
minutes or less. 
The method which M. Janssen has devised with such signal success, 
depends on the fact that the prepared plate is not equally sensitive to all parts 
of the spectrum. In a series of experimental photographs of the solar spec¬ 
trum which M. Janssen had taken in the speaker’s presence, an exposure of 
one-third of a second gave only that portion immediately about the G line ; as 
the exposure was prolonged, the image was extended further in both direc¬ 
tions ; and from two to three minutes gave the whole that could be obtained 
without the admixture of special pigments in the collodion. The first 
condition, then, requisite to obtain a sharp image of the granulations is to 
limit the exposure to the time requisite for the action of the G ray (and 
those rays immediately contiguous). This, in the case of the whole solar 
disk and in a favourable state of the atmosphere, is from _ 1 _to_ a _ nf a 
second. The second condition (seeing that nolens is completelyIchroma- 
tic, that is to say, that no lens has absolutely the same focus for all rays) 
is to adjust the sensitive surface of the plate to the focus of the G ray. 
And the third is to employ a collodion which presents a very perfect and 
even surface. The duration of the exposure is measured and adjusted by a 
very ingenious application of the tuning-fork. A sliding screen with a 
narrow slit is drawn by springs rapidly across the image formed in the focus 
of the objective. The width of the slit can be varied and adjusted by 
means of a micrometer screw. To determine the duration of exposure, a 
™ S l P °^ lass > Previously smoked in a candle flame, is attached by a 
little wax to the slide, and a tuning-fork giving a known note (i. e., givin-r 
a known number of vibrations per second,) with a bristle attached to one 
* On this scale the earth’s disk would bo about 1 inch in diameter. 
