84 REPORT 1871. 



the last two, Nos. 14 and 16, were in all cases marked as " faint," some- 

 times extremely so. 



These seven spots are precisely those which have the highest degrees of 

 visibility for 18 lunations, as under : — 



Spots 1 3 4 17 5 14 16 



YisibiHty .. 1-000 -897 -887 -830 -510 -433 -294 



From these observations, it appears that spots Nos. 1, 3, 4, and 17 may 

 be detected with instruments between 4 and 41 inches of aperture, that spot 

 No. 5 requires an extra half inch, or 4| to 5, and that 5 and 5| will bring 

 out spots No. 14 (5 inches) and 16 (5| inches). 



Aperture, of course, is an important element of visibility; and as these 

 spots are seen with apertures under six inches, as the observations increase, 

 and the normal degrees of visibility become well determined, variations in 

 the visibility of these spots may be detected with instruments of 6 inches 

 aperture, provided the observations extend over a sufficiently long period. 



Elements of Visibilify. 



Lunar. — Brightness and size of spots. 

 Terrestrial. — Clearness and steadiness of atmosphere. 

 Instrumental. — Goodness of figure of object-glass or mirror, and extent 

 of aperture. 



Physiological. — Keenness of eyesight. 



Interval 156 to 144 hours. 



1870, July 14. Mr. Gledhill records No. 1 as a fine, large, bright spot, 

 No. 17 as a small bright spot, Nos. 3 and 30 as bright spots, and No. 5 a 

 bright spot, seen now and then. Mr. lugall records No. 1 as very plain 

 and sharp. No. 4 as steadily seen, and Nos. 3, 31, 30 a misty spot, piobably 

 consisting of these three. 



1869, August 23. Mr. Pratt records that "spots Nos. 1, 3, 4, 17, 6, and 14 

 were very bright compared with their usual appearance, and aU easily seen. 

 No. 4 was not well defined; there was a persistent oval light round it (N.W. 

 and S.E), and I several times believed it to be double, but could not be positive 

 it was so. So remarkably clear was the vision that several times as many as 

 four or Jive spots were held in view at once, without looking directly for 

 them, and two or three times as many as six were so seen, viz. Nos. 1, 3, 

 4, 17, 5, and 14 ; again, Nos, 1, 3, 4, 17, 6, and 5. Nos. 4, 7, 6, 17 were 

 a group seen together, and Nos. 5, 14, 22, and 1 were a similar one ; yet 

 still so exceedingly delicate are the fainter spots and the fainter traces of 

 light on the floor that it needs a most concentrated attention to see cither. 

 In looking for the faint spots the faint traces of light will escape notice ; 

 again, when looking for the latter, the former are most likely not to be 

 seen. This exceeding delicacy too interposes a serious difficidty in cdigning 

 them with objects on or near the border : the eye cannot hold so wide a 

 view and at the same time retain a sufficiently correct impression of objects 

 at once so faint and small. These remarks do not apply to the easier spots 

 and light streaks. Once, for a few minutes, a narrow, dark, straight line, 

 like a pencil-mark, was visible from m towards liamlleta [i. e. from N.W. 

 to S.E.], probably the crack Mr. Birt has discovered. It was not seen 

 again this evening." 



1870, September 11. Mr. Nelson records No. 1 as very distinct, No. 3 as 

 distinct and brilliant, Nos. 5 and 14 as faint, 5 as rather so. 



