300 REPORT— 1872. 



striking." At a still later period Mr. Neison observed Plato from 8.5 to 8.35. 

 He described the floor as " very dark," and gave two gradations of shadow, 

 that in the south-west being marked " dark shadow," while the portion be- 

 yond, towards the east, he marked " shadow." This portion is drawn as ex- 

 tending nearly to spot No. 17, bisecting spot No. 1, and passing a little west 

 of spot No. 16. On comparing the drawing with Mr. Elger's at an earlier 

 epoch, it would appear that Mr. Nelson's " shadow " was in some way con- 

 nected with the streakiness observed by Mr. Elger ; for by 8.35 the true 

 shadow must have retreated to about the position given by Mr. Neison for 

 his " dark shadow." The outline also of Mr. Nelson's " shadow " is not in 

 accordance with the peaks on the west border. Mr. Neison further described 

 the northern light-streak and sector as very distinct and of a pearl-grey 

 colour, and spots Nos. 3 and 17 of a pale grey colour, which he saw di- 

 stinctly. Spots Nos. 16 and 25 are described as " faint." " Although," says 

 Mr. Neison, " this is extremely early, the spots were extremely plain." Mr. 

 Elger's remark is as follows : — " Sunrise finely seen ; shadows of peaks y, 2, e 

 very shai-ply defined ; no spots observed."' 



The darkness of the floor is alike recorded by myself and Mr. Neison ; and 

 Mr. Elger's drawing testifies equally to it, especially on the north-east portion. 

 It is this 'darkness, so unusual at sunrise, combined with the difficulty of 

 making out the streaks and shadows on my part, and the obseiTation by 

 Mr. Elger of the peculiar streakiness, so unlike the ordinary light-streaks on 

 the floor, that lead me to suspect that on the 20th of November, 1871, 

 between 4.35 and 7.40 G. M. T., the interior of Plato was m. an abnormal 

 state. 



While the above-recorded observations were in progress, and the difficulty 

 of observing the interior of Plato from 5.50 to 7.15 was very great, Mr. Pratt 

 observed a very remarkable phenomenon on the Mare Frigoris, which he de- 

 scribed as one of the most singular and striking of all the local observations 

 he had witnessed. The following is an extract from his observing- book : — 

 " 5.30. On a, general survey of Plato and wide neighbourhood, the very pecu- 

 liar aspect of the Mare Frigoris attracts attention. The appearance can be 

 compared to nothing but a kind of haze, entirely local, hanging round the 

 N.W. foot of the slope of Plato. It is the more conspicuous as nothing of 

 the kind is visible either on the Mare Imbrium or on the Mare Serenitatis. 

 The objects on the Mare Frigoris were indistinct, as if veiled. At 6.30 the 

 appearance was much modified. At 7.30 very little of the veiling was to 

 be seen. Between other observations frequent attention was given to it 

 until 9.0, but no return of the phenomenon appeared." 



The contemporaneity of my own observations of the interior of Plato with 

 those of Mr. Pratt of an immediate contiguous locality, is conclusive of the 

 connexion between the abnormal condition of Plato and the veiled appearance 

 of the N.W. slope, extending to the Mare Frigoris. On other occasions 

 Mr. Pratt has described the appearance of the floor of Plato as if seen through 

 a veil of thin white polarized clouds, such as appear in our own atmosphere. 

 Phenomena of this kind are strikingly in contrast with an appearance which 

 I witnessed on the same evening and at the same time, — it was the sharp 

 and weU-deflned character of the broad band of roughened ground extending 

 from the Apennines to the region of Ukert, Pallas, and Bode. 



Such observations as the foregoing remind one strongly of similar observa- 

 tions recorded on numerous occasions by Schroter, which are said by the 

 greatest Selenographer of the present century to have been proved to have 

 been illusions. It is a remarkable fact, and one well worthy of deep con- 



