OBSERVATIONS OP LUNAll OBJECTS. 259 



irregular surface painted with clifFfrent colours presenting an equally diversi- 

 fied appearance, -whether the incident light were oblique or vertical, and 

 the distinction of colour remaining so long as sutHcient light existed to sho-w 

 it, I would remark that there can be no doubt that the moon's surface is as 

 much variegated with colour as the earth's, but by distance the distinction 

 of colour is softened down to tones of grey, in the same manner as wc are 

 able to distinguish nothing but greys in a distant terrestrial landscape. It is 

 the telescope which brings out the distant red-brick building or the dazzling 

 whiteness of the church steeple under a noonday sun, the predominant 

 colour of the landscape being either the delicate green of spring, the deeper 

 green verging on blackness in summer, or the rich reds, browns, or yellows 

 of autumn. These are the colours which characterize the foliated covering of 

 the earth, interspersed with a sandy or even white tint indicative of the ex- 

 istence of vast desert tracts. At the distance of the moon we only perceive 

 on her surface various tints, from a dark blackish grey to a dazzling white ; 

 and these are certainly intensified under vertical illumination, but most 

 decidedly under that reflecting angle the value of which is measured by the 

 supplement of the difference of longitude of the moon and sun when it is 

 equal to zero, or supplement d -0=0°. Perhaps the following experiment 

 may set this matter in a clearer Ught. Take an ordinary cream-coloured 

 envelope and place within it a piece of bluish paper, so that the two tints 

 may appear in juxtaposition, also a piece on which various shades of grey 

 have been dabbed, as trials used in colouring. If these are held or placed in 

 such a position that very oblique light may fall upon them from a lamp, 

 although the distinction of colour may be perceptible, it will be, under the 

 earliest illumination, so very slight as to be hardly cognizable if viewed from 

 an angular position equal to the supplement of 90° : i. e. let the lines from 

 the lamp to the illuminated surface just grazed by the incident rays and 

 from the same surface to the eye form an angle of 90° ; now let the lamp, 

 eye, and illuminated surface be brought into the same plane, although not 

 into the same line, and it will be found that the tints become much more 

 distinct. No more light falls upon the surface than before ; but the eye views 

 the surface under a difi'erent disposition of the angles of incidence and reflec- 

 tion, the consequence being a better appreciation of its inherent light and 

 shade. By placing the diflferent shaded papers in such a position that the 

 light from the lamp falls perpendicularly upon them, and bringing the eye as 

 nearly as possible into the same perpendicular line, we view the paper as we 

 view the full moon, the tints coming out in the strongest manner possible ; 

 and this is in accordance with the law that the greatest quantity of light is 

 irregularly reflected with the smallest angle of incidence. As the diameter 

 of the moon subtends a maximum angle of less than thirty -three minutes of 

 arc, the rays coming or reflected from her are nearly parallel ; from which it 

 follows that the path of the solar rays impinging on the moon, and passing 

 to the earth, will be nearly as the sides containing the angle known as the 

 supplement of the moon's elongation from the sun, which at full ecuals 0°. 



In applying our experiment to the moon in all its generality, we ought to 

 have a regular increase and decrease of intensity of tint, subject to small but 

 also to very regular variations. Is it so? In one remarkable and well- 

 observed instance, at so early a period as twenty-four to thirty-six hours 

 after sunrise on Plato, the north-west portion of the floor was so strongly 

 illuminated as to obliterate the well-known north-west streak. This appeared 

 to be an abnormal brightening of the floor, and must have been qmte inde- 

 pendent of illuminating or reflecting angle ; its bearing upon Mr. Pratt's. 



T 2 



