HARD WICKE'S SCIENCE- G OSS I P. 



1 1 



of the full moon has been found to be equal to that 

 of a candle at a distance of rather more than 8 feet, 

 or in other words it would require 70 full moons to 

 afford as much light as the sperm candle does at the 

 distance of one foot from the eye. The law by which 

 the amount of lunar light increases or diminishes with 

 her changing phase is a somewhat complicated one, 

 nor can it be said to have yet met with an exact 

 mathematical expression, though more than one close 

 approximation has been made. That which best 

 satisfies the case is attributable to the German mathe- 

 matician Lambert, but it is based on an assumption 

 that the moon, like the planet Jupiter, or the sun 

 itself, is brighter at the centre of the disk than round 

 the periphery. Owing either to the material con- 

 stituents of the moon, or more probably to the con- 

 figuration of the surface, this is not the case, and 

 hence the want of exact correspondence between 

 theory and fact. It will serve, however, to give us a 

 pretty accurate idea of the amount of light derived 

 from the moon at her principal phases. Thus, when 

 horned, i.e., midway between the new moon and the 

 quarters, the total illuminating power is barely one- 

 iwentieth of its brilliancy at full ; at the quarters, 

 when one-half of the illuminated side of the moon is 

 turned towards us it has increased to very nearly one- 

 third ; and when gibbous, or half-way between the 

 quarters and the full moon, fully three-quarters of its 

 maximum light reaches the earth. It is during the 

 two or three days after the first quarter and before 

 the last quarter that the change in the moon's bril- 

 liancy is most rapid, and near the new and full that 

 the variation is the least. 



If the moon reflected all the light which she receives 

 from the sun there would still be a great disparity 

 between these luminaries, and it would require as 

 many as 45, 193 moons to equal the sun's light, but 

 in fact she is much less generous to us and the dis- 

 parity is very considerably greater. The earlier 

 attempts at a comparison of their respective lustres 

 made by Bouguer and Wollaston by no means con- 

 firm each other, and even the modern measurements 

 of Zollner and Bond are less accordant than we could 

 wish. If we assume that Bond's results are most to 

 be trusted, the sun is no less than 470,980 times 

 brighter than the moon, from which it will be at once 

 inferred that the latter returns to us less than a tithe 

 of what she receives, absorbing fully nine-tenths for 

 her own benefit. Bond has shown, however, that in 

 reflecting the actinic rays she is proportionally more 

 generous, returning to us nearly a seventh. It is this 

 selective power of reflection which causes the differ- 

 ence of colour in the light of the two bodies, the 

 excess of violet and ultra-violet rays which she reflects 

 •converting the yellow tinged solar light into that of 

 the silvery moon. Moreover it seems possible that 

 while sending us an excess of the more refrangible 

 rays, the less refrangible or heat rays are in defect, 

 for it is well known that we derive no heat from the 



moon whatever or the most infinitesimal quantity. 

 It has been suggested that the upper regions of our 

 own atmosphere would absorb all the heat reflected 

 to us by the moon, still the analogy would point to 

 this not being the only cause of its entire absence at 

 the earth's surface. 



The brilliancy of the sun follows from the fore- 

 going remarks to be equivalent to 6683 sperm candles 

 at the distance of one foot from the eye, a number 

 more than 1000 greater than what is usually given 

 upon the authority of Wollaston ; but there is some 

 difficulty in reconciling this philosopher's results with 

 one another, and it is a clear indication of the slight 

 extent to which this obscure page of science has been 

 read that his authority should still be so frequently 

 quoted. 



We will now turn to the lesser lights of the firma- 

 ment and learn how much less bright these are than 

 the moon, which will thus serve as a connecting link 

 between them and the great light-giver of the solar 

 system. Until recent years it had been found im- 

 possible to compare these minute points of light 

 either with one another or with any standard source 

 of light, artificial or natural, and consequently rough 

 estimates were made, throwing the stars into classes 

 or magnitudes according to their relative brightness 

 as judged by the unaided eye. As time went on 

 these rough estimates began to define themselves 

 more and more sharply and to be subdivided, until 

 the system was perfected and stereotyped as it were, 

 so that what required to be done was merely to com- 

 pare instrumentally the photometric intensity of these 

 arbitrarily assumed magnitudes. It is now found (by 

 the aid of Zollner's photometer, which taking advan- 

 tage of the properties of polarized light, has rendered 

 the comparison possible) that a star in one of these 

 classes possesses almost exactly two and a half times 

 the amount of light of a star in the class next below 

 it, and consequently a star of the sixth magnitude, 

 which is the faintest that can be seen by the naked 

 eye, is equivalent to J m part of the light of an average 

 first magnitude star, or 3 g S5 part of the light of 

 Venus at her greatest brilliancy, or 33555153 part of the 

 light of the full moon. There is further reason to 

 believe that the whole of the stars visible upon a fine 

 night, collectively afford as much as -^ part of the 

 light of the full moon, an amount which, I believe, 

 they have seldom had the credit of supplying to the 

 service of man. 



Mistletoe. — Before your list of the various 

 habitats of the mistletoe is closed, I would record an 

 instance of its growing on the horse-chestnut, which 

 for several years past I have observed in Herefordshire. 

 It is on a young tree in the garden of a labourer's 

 cottage situate at Southfield, about two miles below 

 Bosbury, on the east side of the river Leadon. — 

 Vincent S. Lean. 



