140 H. F. Blanford— M. Janssen's Photographs of the Sun. [Apeeg, 
of its prongs, is set vibrating transversely to the path of the slide. The 
slide being then released, a waved line is engraved on the smoked surf ace ; 
and supposing that the fork vibrates 500 times in the second, the length oi 
each wave represents the distance traversed in ^ of a second. It then 
the width of the slit be adjusted to one-sixth of a wave length, the duration 
of the exposure afforded will be -3 oV o a secon( ^- • 
In conclusion, Mr. Blanford remarked on the wide field for the study 
of solar phenomena which is opened up by M. Janssen’s discovery. The 
sun is still passing through a prolonged period of minimum activity, indi¬ 
cated hy the absence of spots, faculai and hydrogen flames. It is now in 
a state of comparative quiescence. Mr. Norman Lockyer has particularly 
remarked the reduced size of the corona, the absence of hydrogen flames and 
the predominance of the characteristic lines of magnesium* m the 
chromosphere of the late eclipse. It will be especially interesting to ob¬ 
serve the changes presented when the sun approaches another period of 
maximum activity, since facute always accompany sun-spots. It may be 
safely predicted tfiat the brilliant faculse and hydrogen flames (the same 
phenomenon seen from two different points of view) will increase at least 
to the same extent as the spots, and thus will probably be explained the 
apparent paradox which results from measurements (such as we have) 
of the solar radiation, viz., that the sun is hottest when the spots are most 
numerous. It is a common popular idea, that the reverse is the case. Igno¬ 
ring direct observation and reasoning a priori , it is inferred that the 
spots being cooler than the luminous disk, the sun must emit less heat 
when these are most numerous. It is with the character of the sun’s sur¬ 
face as with that of men. The eye seizes quickly on the blots, but ignores 
and takes no note of the brilliant faculai which accompany them, and out¬ 
shine the general surface. 
Capt. Waterhouse observed, with reference to Mr. Blanford’s state¬ 
ment that the photographs of the sun previous to those done by M. Janssen 
were quite blank and showed no traces of structure beyond the spots, that 
when exhibiting one of M. Janssen’s photographs at the Meeting in May 
last, he had mentioned that some of the small photographs taken with the 
photoheliograph by Colonel Tennant at Koorkee after the Transit of \ enus 
showed very distinct traces of structure over the solar disc, and though of 
course they were wanting in the fine details of the granular structure, the 
outlines of what he believed M. Janssen called the polygons of his network 
were easily distinguishable. These photographs had been very carefully 
* Mr. Loclryer’s view as is well known is that the greater intensity of the solar 
heat at the timo of maximum sun-spots, causes a dissociation of the magnesium molo- 
culi, resolving it into hydrogen. 
