292 TRANSACTIONS OF SECTION A. 
In September 1913 a tower telescope, forming the image of the sun by the use 
of mirrors without lenses, and yielding an image of about 9 inches in 
diameter, was made ready in connection with the station of the Smithsonian 
Astrophysical Observatory on Mount Wilson. The image of the sun is caused 
to fall upon the slit of a spectrobolometer, which slit for this purpose is only 
about three-eighths of an inch in height. By stopping the clock of the telescope 
the solar image drifts centrally across the slit of the spectrobolometer, owing 
to the diurnal rotation of the earth. There is thus produced, by automatic 
registering of the indications of the bolometer, a curve of distribution of in- 
tensity along the diameter of the sun’s disk. This curve takes the form of a 
letter U. The length of the straight sides of the [J may be taken as represent- 
ing the intensity of the solar radiation at the edge of the sun’s disk, and the 
height of the U to the centre of the curve may be taken as representing the 
intensity of the radiation at the centre of the disk. Thus a contrast of the 
intensity of radiation along the diameter of the sun is made manifest. Obser- 
vations were made on nearly fifty days of the year 1913 with this apparatus, and 
on each day the distribution of intensity at seven different wave-lengths of the 
spectrum between 3,700 fingstroms and 10,000 &ngstréms was determined by 
making two curves at each wave-length. On the same days the solar constant 
of radiation was determined at Mount Wilson. 
Work with a similar object, but done in different ways, has been carried 
on by Vogel, Pickering, Langley, Very, Schwarzschild and Villager, and Abbot 
and Fowle. It is found by comparison of the distribution curves obtained at 
Mount Wilson in 1913 with others obtained by Abbot and Fowle in 1907, that a 
change of form of the distribution curve has occurred between these epochs. 
The contrast of brightness between the centre and edge of the sun in the year 
1907 was greater than the contrast found in the year 1913. This is verified at 
all wave-lengths, but the change of contrast is greater for short wave-lengths 
than for longer ones. 
It further appears, by comparison of results of one day with another in the 
year 1913, that a change of contrast of brightness is going on all the time, 
similar in irregularity of period and amplitude with the variation of the sun’s 
total radiation which was found by comparison of Mount Wilson and Bassour 
observations. When the daily values of the solar constant of radiation obtained 
in 1913 are compared with the distribution of brightness along the sun’s 
diameter, it is seen that a close correspondence of variation occurs between the 
two. This daily variation is of such a nature that when the solar constant values 
increase the constant of brightness between the centre and edge of the sun 
diminishes. The result is contrary to that which was indicated by a few 
observations of Abbot and Fowle in the year 1908. It is believed on further 
examination that the results of Abbot and Fowle in 1908 were made erroneous 
by certain defects in the measurements of the solar constant of radiation on two 
or three days. The new results come from nearly fifty days of observation, and 
are quite definite in showing the connection between the variation of the radia- 
tion of the sun and the variation of brightness along the sun’s diameter. 
It appears, however, that the correlation between solar constant values and 
contrast values between the years 1907 and 1913 is contrary in its sign to the 
correlation between these variables exhibited by the daily march of values for 
the year 1913. This may point to a greater complexity of the solar problem 
than was at first indicated by the results of Abbot and Fowle. It may be that 
the march of the sun-spot period attends an influence in one direction, while the 
march of short-period fluctuations of the solar radiation from day to day attends 
a change of contrast in the other direction. 
4. Discussion on the Present State of the Problem of Australian 
Longitudes. Opened by P. Baraccui. 
In Mr. Baracchi’s paper were discussed the longitude values assigned to the 
two Australian meridians of Port Darwin and Southport (Queensland); these 
being, respectively, the terminals of the two chains of telegraphically deter- 
mined longitudes carried eastward from Greenwich via India, Singapore, and 
