312 PROFESSOR L. BECKER ON THE DISTRIBUTION OF 



diffused light. The left aud right halves of a photograph will be designated by a 

 and b. Photographs I. to VII. occupy the first half-plate, and VIII. and IX. half of 

 the second half-plate. 



Photographs VI. and VII. are, from an ideal point of view, marred by defects 

 No. VI. by some instantaneous pictures of the protuberances which appear on the 

 lunar disc, and No. VII. by two short exposure pictures which are eccentrically 

 superposed on it. These defects are of no consequence (see 6). Owing to the 

 failure of the automatic apparatus, the exposures of Nos. VI. and VII. are uncertain 

 to about a second, but the sum of the two exposures, which equals the sum of the 

 sixth, seventh and eighth exposures, is accurately known. 



To get the negatives, from which the Plate was prepared, I first made enlarged 

 positives, copying those pictures together whose background have the same density 

 on the original. I have attempted to make the coronas of Via. and VI6. extend 

 equally far, and in the attempt the coronas of pictures V. and VIII. have come out 

 too small. I should say that on the original negative the corona of VIII. covers the 

 whole breadth of the plate, which towards the top is three-quarters of a solar 

 diameter broader than shown in the reproduction. The enlargement is 1*7. 



3. The Measurements. 



The observations made on the photographs, and utilised in this paper, consist in the 

 selection of points on the several corona pictures at which the photographic film shows 

 the same degree of blackness, and in the measurement of their distance from the lunar 

 disc. The measurements were actually made on positives, and not one but twenty- 

 four points of an equal-blackness curve, 15 degrees apart, were measured. The 

 positives are contact prints on slow plates obtained at a distance of 10 feet from a gas 

 jet. The twelve sets which I prepared belong to different exposures, and were 

 developed for contrast. I copied the negatives I. to Va., Vt. and Via., VI6. and VII. 

 separately on account of the differences in density of the background. Sixteen sets of 

 measurements of equal-density curves were made on these twelve sets of positives. 

 The measurements were easy to make, and proved to be consistent. The positives 

 show a perfectly transparent ring round the lunar disc, the diameter of which depends 

 on the exposure and development. Seen against black paper, this ring furnishes a 

 well-defined outline to set upon. Some of the curves at great distance from the sun, 

 where the intensity changes little with the distance, were measured on enlargements 

 (10 diameters) on bromide paper, in which the contrasts are much increased. 



I further made twenty copies of each of Photographs VIII. and IX. at all kinds of 

 exposures. These negatives are very dense, and, apparently, evenly dense up to 

 about half a diameter from the sun's limb, and show no detail to the eye when 

 inspected against a strong light. On the other hand, the positives contain detail as 

 near as 0'12 diameter, and as distinctly as if they were replicas of the first six 



