Apparent Size of the Moon. 391 



than horizon, and, secondly (B), the apparent distance of the 

 luminary accords with that of the sky. 



A. Everything visible is seen by rays proceeding from it*. 

 Whatever, then, constitutes the visible sky is thus seen. Now 

 sky would appear black if atmospheric particles did not reflect 

 rays; it would appear white if intervals of black space beyond 

 did not leave points of the retina unexcited ; and it appears blue 

 by a law (experimentally deduced by Da Yinci) that retinal 

 excitation (by white light) interspersed with retinal points of 

 rest (derived from the blackness of space) is the sensory condition 

 blue. But the (partial) blueness of sky at horizon shows that 

 the blackness of space is not altogether shut out by intervening 

 reflecting particles: whence it follows that the furthest reflecting 

 particles of our atmosphere cannot have their light wholly inter- 

 cepted by the nearer ; and that the whole atmospheric area contri- 

 butes, with the blackness of space beyond, to constitute the visible 

 sky. The whole atmosphere, then, gives the real locality of that 

 we call visible sky — seen by rays proceeding from it (its varying 

 shades, from blue to nearly black, in various latitudes, showing 

 that colour is its visible accident and not its visible essence). Now, 

 whether we draw a line from the spectator to the limit of the 

 atmosphere or to any assumed point within it, the line in direc- 

 tion of horizon will be longer than in that of zenith; and in 

 whatever form any part, or the whole, of this transpicuous extent 

 of atmosphere is manifested to vision by the aforesaid rays, such 

 manifestation in direction of horizon is more or less obscured, in 

 consequence of the rays depicting it coming from the greater 

 distance and under a smaller angle ; in other words, every visible 

 quality and attribute of the sky will appear more distinct at the 

 nearer point (zenith) than at the further point (horizon) f. 



But if this necessarily curtailed demonstration should not be 

 fully conclusive, the facts, reasoned to, are obvious ; the sky is 

 more definite and more blue in appearance at the zenith : more- 

 over we naturally express our sensation by Darning it the "near 

 sky," and that at the horizon the "far sky." The synthetic 



* For, though perfect blackness, with its retinal equivalent non-excite- 

 ment, is often an instrument of vision, an object from which no rays pro- 

 ceed is, per se, invisible. 



t Another source of obscuration, owing to the greater densitv of the 

 lower strata of atmosphere (a consideration that has misled some writers 

 on this subject), is intentionally omitted, as also (further on) obscuration 

 of the luminary itself, from the same cause. For, first, the senses (which 

 are much more wakeful observers than the reason) are conversant with the 

 fact of the bases of high buildings, trees, vertical cliffs, &c. being more 

 obscured than their summits, in which case the sense knows that greater 

 obscuration does not mean greater distance ; secondly, my second experi- 

 ment, already stated, proves that our phenomenon is, or may be, independent 

 of this obscuration. 



