134 



♦ KNO\A^LEDGE ♦ 



[Aug. 31, 1883. 



the object is increased in a greater proportion than the 

 light. Therefore the apparent brightness of the object is 

 diminished. 



Lastly, let us suppose n n' greater than d. Then the 

 magnification of areas is, as before, represented by the ratio 

 (OO'f 



(n n')' 



and the increase in the quantity of the light falling 



on the eye is as (0 0')" to d' ; but this quantity does not enter 

 the pupil, because it comes out in a pencil of parallel rays 

 having a cross-section of diameter n n', and the pupil has 

 only a cross-section of diameter d ; the proportion ent(!ring 

 the eye is tlius reduced in the proportion of the areas of 

 these cross-sections, or as d- to {nnf. Hence the quantity 

 of light actually received by the pupil is represented by the 

 proportion 



(0 O'f d- ^^ (O O'f- 

 d? {ii n)- [n n')'~ 



which is the proportion in which the apparent area of the 

 object is increased. Hence the brightness remains un- 

 changed, as in the first case. The only difference, in fact, 

 between this case and the first is that the telescope is not 

 in the last case pushed to the full power it will bear with- 

 out a reduction in the apparent brightness of objects : in 

 fact, only a portion of the object-glass is really used in 

 bringing light to the eye from any point of the object. Light 

 from each point of an oliject, in such a case as this, em- 

 ploys (as it were) its own part (a circular part) of the 

 object-glass.* 



Of course, the above reasoning does not apply in the case 

 of a star, because a star is not apparently magnified by a 

 telescope ; but on the contrary, owing to the operation of 

 laws with which we are not at present concerned, the 

 image is reduced. Thus, the brightness of a star is in- 

 creased pretty nearly in proportion to the area of the 

 object-glass or mirror. 



But even in the case of stars the law above considered 

 has its analogue. Take an imaginarj' case, where (say) 

 twenty stars form a cluster (each star being visible 

 to the naked eye) and the telescope shows those twenty 

 stars and no more ; then, the apparent brightness of the 

 cluster, regarded as a whole, remains unaltered when the 

 telescope is used ; for in precise proportion to the increase 

 in the brightness of the individual stars is the increase in 

 the apparent area of the cluster.! 



The application to the planets, moon, sun, kc, is 

 obvious. 



It may be asked why night-glasses show objects which 

 the unaided eye cannot see ; and again why, by the use of 



* The case corresponds to thut of an opera-glass (for it is easily 

 seen that the reasoning would not be affected by substituting a 

 Galilean for a simple astronomical telescope). Accordingly it will 

 be found that in using an opera-glass we can hide or dim a' portion 

 of the field of view by covering a part of the object-glass. But in 

 the case of a telescope used (as the astronomical nearly always is) 

 with its full power, covering a part of the object-glass dims the 

 whole field equally. Dr. Huggins told me, six or seven years ago, 

 of an old telescope he bought, in which the object-glass was four or 

 five inches in diameter, and the focal length little more than a foot. 

 The emergent pencil was an inch or so in diameter (with the 

 power actually employed). As Dr. Huggins quaintly remarked, 

 it was " a telescope for a horse, not for a man." 



t In passing, I may notice that the same law applies to the 

 effects of distance. A cluster of stars, separately discernible, would 

 remain just as bright regarded as a whole (thatis, not considering 

 the total quantity of light received from it, but its brightness as 

 related to its area), however far it might be removed, so long 

 as the separate stars continued discernible. Nor would it grow any 

 brighter, as a whole, however near it might approach to the observer. 

 (It is assumed here for the moment, that there is no extinction of 

 ] ight in the interstellar spaces.) 



single magnifying glasses we can read at dusk what would 

 be undiscernible, or only discerned with difficulty, by the 

 naked eye. The answer simply is that the quantity of 

 light received from the object is increased. It is easy to 

 test this. 



Let the observer look at the page of a book with a 

 magnifying glass in the dusk of evening, and notice 

 whether the part seen with the glass (and rendered legible 

 by its means) seems a brighter circular spot on the darker 

 ground of the rest of the page, or vice rers<1. If he is care- 

 ful not to let the diffused light from the glass reach his eye, 

 he will see at once that the legible part of the page looks 

 darker than the part outside, which nevertheless is 

 illegible. 



It is equally clear, but the point requires to be more 

 carefully dwelt upon, that we cannot by increasing the 

 size of telescopes increase the apparent brightness of such 

 objects as nebula^, comets, the zodiacal light, kc. In the 

 case of very large objects like the zodiacal light we cannot 

 even increase the total quantity of light received by the 

 eye. In the case of comets, nebulre, &c., we can do so; 

 and so we can render objects of this sort discernible, or 

 even conspicuous, which otherwise would remain unseen. 

 But it is important to notice that we cannot increase their 

 brightness. It is this which constitutes one of the great 

 difficulties in applying the spectroscopic analysis of these 

 objects ; for in such analysis it is often convenient to use a 

 rather narrow slit, and therefore the spectroscopist cannot 

 avail himself of the great quantity of light gathered up for 

 him by his telescope. Some astronomers, therefore, in 

 examining the nebulaj which give three or four bright- 

 line spectra, use an open slit, or no slit at all, getting 

 three or four images of the nebula, instead of three or four 

 lines. 



In another kind of research — the study, namely, of those 

 faint but widely extending nebulosities which Sir W. 

 Herschel was the first to notice — a large telescope is of no 

 great use. I am persuaded, indeed, that the naked eye can 

 do more in work of this sort than is commonly supposed, 

 care being taken to protect the eye from extraneous light, 

 and also to conceal the lucid stars from view. 



Autumn Leaves. — Messrs. Sampson Low, Marston, k 

 Co. are issuing for the autumn holidays a new edition of 

 Mr. F. G. Heath's "Autumnal Leaves" — a work which 

 the author claims to be the only one ever published in 

 Europe or America giving actual facsimile representations 

 in colour and venation of autumn-tinted foliage. 



A Cakbonaceous Meteorite. — An interesting meteorite 

 which fell in the province of Entre-Rios, Argentine Re- 

 public, has been examined by M. Daubrce. The meteorite 

 recalls certain kinds of lignite and clay coals, such as the 

 boghead coal. In the black paste of the meteorite are to 

 be seen small angular grains of a bottle-green of a hyaline 

 colour, others again are whitish. The whole appearance of 

 the stone resembles some of our volcanic conglomerates. 

 Brass-yellow grains like pyrrhotine are also visible, and 

 reddish spots like the chloride of iron sometimes seen in 

 meteorites. Chemical analysis shows that the meteorite 

 contains iron, lime, and magnesia. The most important 

 peculiarity of the meteorite is, however, that it contains 

 carbon in an organic form. This is chiefly proved by the 

 action of potash on it. In fact, Dr. Daubrce hopes yet to 

 find a meteorite bearing organic remains. Other carbona- 

 ceous meteorites have fallen in divers places, but none, so 

 far as we are aware, have yielded traces of organic life. — 

 Engineering. 



