Aug. 31, 1883.] 



♦ KNOWLEDGE ♦ 



133 



Central Telegraph Office in London. Were the resistance 

 of the circuit low, polarization would speedily ensue. 

 Nevertheless, the agglomerate cell is to be preferred to 

 the ordinary Leclanchu, even for amateur purposes. The 

 blocks are very durable, and after nearly two years' hard 

 service show little or no signs of giving way. 



It is still an open question whether the agglomerate cells 

 have any advantage over the parent form, other than is 

 derived from the absence of the porous pot and the increased 

 zinc surface. Consequently an excellent form is that 

 known as the Grenet, in which the porous pot is replaced 

 by a stitched canvas bag to hold the manganic peroxide 

 and carbon. In this instance all the gain pertaining to 

 the agglomerate is embraced. 



The Bennett cell is in principle somewhat akin to the 

 Leclanche, but for general work is not to be compared with 

 it. The positive element is a piece of zinc immersed in a 

 solution of caustic soda, a compound which has the merit 

 of being not only inexpensive but equally nasty. A porous 

 cell containing the soda solution is stood in a tin, or rather 

 tinned-iron, can, the space between the can and the porous 

 pot being filled with iron turnings. The solution gradually 

 penetrates the porous pot, and, damping the turnings, a 

 circuit is formed and a current generated. This current is, 

 however, very spasmodic, and even less constant than 

 that from the Leclanche. It can, therefore, found no 

 great claim on its efficiency, while on the other hand it 

 enjoys the privilege pertaining to all batteries containing 

 iron, of perfuming the air with sulphuretted hydrogen — 

 an odour which, if it is like anything on this earth, 

 resembles most strongly the delicious aroma of rotten 

 eggs. When the cell was first introduced, accompanied as 

 it was with loud and general acclamations, it was recom- 

 mended as being extremely cheap in construction, the outtr 

 or containing vessel being an old preserved meat-can. We 

 noticed, however, that those sent out by the company which 

 enjoyed the privilege of selling it, were contained in spe- 

 cially made tins, having a closely-fitting cover, presumably 

 to bottle up the scent. It is manifest from what we have 

 said concerning the cell that it is impossible for us to re- 

 commend its adoption for any purpose whatever. 



We shall next turn our attention to a large class of bat- 

 teries, embraced under the generic appellation of " Bichro- 

 mate," and amongst them we hope to see some of exceptional 

 utility to the student. 



LAWS OF BRIGHTNESS. 



IX. 



By Richard A. Proctor. 



IT remains only that I should consider the effect of 

 instrumental appliances for increasing the degree of 

 light which we receive from objects. 



The telescope is an appliance of this sort ; in fact, we 

 may regard as the main feature of the telescope its power 

 as a light-gatherer. In whate\er proi^ortion the object- 

 glass of a refractor, or the mirror of a rellcctor, exceeds in 

 area the pupil of the eye, in the same proportion (neglecting 

 loss of light by imperfect reflection, or through refraction) 

 may the quantity of light received by the eye when the 

 telescope is used exceed the quantity received from the 

 same object when the eye is unaided. I say inai/ rather 

 than does ; for matters may be so arranged that the full 

 light-gathering power of the teUscope is not employed. 



Xow, it seems at first sight thut in consequciue of this 

 relation the apparent briglitncss of an object observed with 

 a large telescope must be enormously increased. And, as 

 I mentioned in the first paper of this series, the idea seems 



monstrous to many that there cannot under any circum- 

 stances be an increase of apparent brightness of an object, 

 though there may be an enormous increase in the total 

 quantity of light received from the object. " Do you mean 

 to tell me," an esteemed friend of mine once remarked, 

 "that the moon is no brighter with my 6i in. reflector than 

 with the naked eye 1 It mngt be brighter. I can scarcely 

 bear to look at the moon, it is so bright, when I use a low 

 power and the moon is high up in a clear sky ; but I have 

 never found any trouble in looking at the moon with the 

 naked eye." Be it understood that there was no question 

 as to the point under discussion. It was the intrinsic 

 lustre of the moon's surface, not the total quantity of light 

 received from her, that we were both considering. I 

 pointed out, though I am bound to admit the argument 

 was not found to be convincing, that there is the same 

 intrinsic degree of pain when one tooth is pulled out as if 

 a dozen were extracted at once, yet a man might easily 

 bear one operation who would faint under the other.* 



But as a matter of fact the case of the telescope is 

 exceedingly simple. Take, for convenience, the simplest 

 form of the astronomical telescope, which gives the brightest 

 images for a given magnifying power. Let O' (Fig- H) 

 be the object-glass, e v the eyeglass ; and let a pencU filling 

 the whole object-glass converge to a focus at F, and after 

 falling on the part mm' of the eye-glass emerge with parallel 

 rays and fall on the pupil of the eye E. Its cross-section 

 is either just equal in diameter to the pupil of the eye, or 

 less, or greater. First suppose it equal. Then the total 

 quantity of light received from the point of the image, 

 exceeds the quantity which would fall on the pupil of the 

 naked eye as (O O')- exceeds (w n'f. But the magnifying 

 power of the telescope is represented by the ratio 



OF , • u • ii, ..00' 

 , or, which IS the same ratio, — . 



F n n n' 



Therefore the apparent area of the object is increased in 

 the ratio (O 0')- to (nn')- — the precise ratio in T which 

 the total quantity of light received from the object 

 is increased. This increase, then, is just competent to 

 make the increased area as bright as the area seen by the 

 unaided eye. 



But next suppose iiw' less than the diameter of the 

 pupil, and call this diameter cl. Then, as the whole of the 

 emergent pencil falls on the pupil, the increase of the total 

 quantity of light is as O O'- to d'-, but the magnifying 



power of the telescope is, as before, ,, and the mag- 



'^ n n 



fication of areas is as O 0"- to (w n'f. Thus the magnifi- 

 cation of areas is to the total increase of littht as 



(O o')- (o 0') - 



{niilf • dr- 

 or as 



d- : (■« n')'. 

 But d is greater than n n' ; hence the apparent area of 



* I suppose this to be the case, but do not speak from experience. 

 I have heard, however, that even a man of the strangest frame 

 would bo shaken if a dozen teetli were extracted in quick succes- 

 sion : aud I presume tlio e-Ktraction of a dozen at the sume instant 

 would be even more trying. By extraction, I mean fair dcliiicrato 

 lugging out, such as the soul of the dentist loveth — not knocking 

 out by a blow; thongh the knocking out of a dozen tep-h nt once 

 can hardly be a pleasant experience. 



