TOTAIi SOLAE ECLIPSE OF JULY 18, 1860. 
399 
July 18, 2^^ 2“ 35® Greenwich mean time, the moon had occulted spot c, at a distance 
of '6061 of the sun’s radius from the sun’s centre, and between the position-angles 
300° 58' and 302° 33', reckoning from the respective edges of the spot. 
At 3^ 11“^ ll®-7 the spot c was partly uncovered by the moon’s edge at a distance of 
•6172, and between the angles 300° 48' and 302° 45'. 
At 3'’ 58“ 58®*3 the moon’s limb had partially passed off spot h, at a distance of ’OOll, 
between the angles 128° 3' and 130° 5'. 
I am not aware that any practical use has ever been made of the occultation of a sun- 
spot by the moon during an eclipse ; but as it is probable that during the eclipse under 
discussion such phenomena were recorded by a great number of observers, as on the occa- 
sion of pre-vdous eclipses, I have thought it deshable to make the measurements given 
in Table V., which will, I beheve, afford better means than have been before available for 
turning such observations to account. 
Photographs of the Totality. 
Copies of the two totahty-pictures which accompanied this paper were produced in 
the following way : the original negatives were placed in the focus of an enlarging-camera, 
and positive collodion copies on glass procured, on which the lunar disk was enlarged to 
9 inches in diameter. These positive copies were then placed in the focus of the camera, 
and a number of negatives made, to print any impressions that might be required, in 
which the lunar disk was reduced to the sizes of the several engravings accompanying the 
paper. The photographic copies therefore are two removes from the original, and, some- 
thing being lost at each operation, they do not present all the details visible in prints 
taken direct from the original negatives. The corona, for example, which is depicted on 
the original negatives, is to a great extent lost in the copies, because in bringing clearly 
out the details of the prominences, the corona in most cases becomes over-printed. 
A few positive 9-inch copies on glass have been presented to Observatories and public 
Societies ; but it was not possible to do this very extensively, in consequence of the extreme 
difficulty of copying, occasioned by the density of the original negatives. They could 
only be procured on days when the sun was perfectly unobscured by haze or cloud, and 
ultimately the injury to the second original negative prevented my continuing the work 
sufficiently long to obtain a supply as great as I deshed. Secondary copies can, however, 
stm be procured. 
In Plate IX. are given mezzotint fac-similes of the two totality-pictures the size of 
the originals ; although they will serve to give a general idea of the photographs, and 
to illustrate what has to be said respecting them, they are, after all, but imperfect sub- 
stitutes for the photographs themselves. Copies of the photographs would have been 
inserted in this memoir, had past experience of the permanence of such pictures war- 
ranted the Council in doing so *. 
* In the Author’s copies, Plate IX. a, photographic copies from the originals are given. They are two 
removes from the originals. 
3 H 2 
