TRANSACTIONS OF THE SECTIONS. 



17 



some reasonable conjectures on the mysterious light streaks which radiate from 

 some of her mountains. 



To what degree of minuteness shall we see the objects ? This question has not been 

 much considered with reference to photography, or the kind of objects which the 

 moon exhibits. If we assume that one minute of angle :s a good general rneasure 

 for the visibility of areas presented to the eye, and therefore that areas are visible at 

 a distance about 3000 or 4000 times as great as their diameters, an area on the 

 moon, 70 miles across, can besee«by the naked eye; magnifying this 1000 times, we 

 may see an area on the' moon t^Su of a mile across, or 370 feet. But though a 

 spot of such dimensions can be seeii, it cannot be defined under such a power as 

 square, circular, elliptical, or triangular. 



To be thus clearly defined, so as to be positively drawn or described, its diameter 

 must be such as to subtend nearly 3' of angle ; so that to be clearly defined to the 

 naked eye, black spots on a white ground must have a diameter of about j^ of 

 radius=200 miles, and under the magnifying power of 1000, 4rT=-r of a Di'le 



^ lOUO 5 



= 1056 feet. 



But this calculation applies to black spots not greatly varying in their diameters. 

 We have on the moon many cases of entirely different figures, arched, or 

 triangular shadows, long streams of light, and long stripes of darkness. I was 

 much impressed while at Parsonstown with the minuteness of some of the ' rillen,' 

 as the Germans call the narrow deep often winding clefts, such as those about 

 Aristarchus, and the much finer ones on the north-east of the Mare Humorum, of which 

 I have made drawings. On returning home, I made some trials of the visibility of 

 narrow spaces, as compared with square areas of the same breadth. The results, which 

 are of a kind to encourage greatly our surveys of the moon, appear in the sub- 

 joined table, and indicate that black narrow spaces not exceeding 12 feet in width, 

 are within the magnifying power of the great Rossian reflector. To what extent 

 the photographic power of the instrument is competent to define such shadows, 

 or the mechanism which must be employed to follow them exactly, are points for 

 experiment to settle. As far as the eye is concerned. Lord Rosse's mirror has light 

 enough for such a power, but the eye is more sensitive than collodion. 



Hence it appears that linear spaces may be noted as such to three, five, ten, and 

 even thirty times the minuteness with which spots can be well defined. The 

 distinctness of very narrow ' rillen' is thus accounted for, but at the same time 

 it appears that the breadth they seem to bear is merely the ' optical,' not the 

 ' physical' breadth. If we apply the last measure to the moon, we find that very 

 narrow and very dark spaces (' lines' in ordinary language), less than seven miles 

 across, on the moon, would be visible to the eye, through a really ' clear' atmo- 

 sphere. By applying to a small portion of the moon, Mr. Dawe's process of 

 scrutiny by small apertures, and a power of 1000, black bands 12 vards across might 



1853. ' 2 



