1882.] in twilight 207 



1776 : and of the certainty of which he can have had no hesitation 

 as he says : " this observation completely refutes an objection to 

 telescopic vision that has been drawn from what has been demon- 

 strated by optical writers, namely that no telescope can shew an 

 object brighter than it is to the naked eye." I will now offer two 

 possible explanations which have suggested themselves to me of a 

 phenomenon as to which that eminently practical observer had 

 evidently no doubt. 



One of them is based in some degree on what the younger 

 Herschel says as to the way in which a telescope actually renders 

 the smallest stars visible. To use a simple argument, he takes a 

 star as a point which has no magnitude, and infers that however 

 the small section of the sky on which the eye is for the moment 

 concentrated is magnified by the telescope, all the augmented 

 light received from that point through the object-glass is accumu- 

 lated in one single point : which thus, though invisible to the 

 naked, is clearly discerned by the assisted eye. It seems to me 

 possible that the way in which the retina of the eye becomes sus- 

 ceptible of light under the circumstances referred to, is analogous 

 to this: viz. that the eye practically receives the light only at what 

 we may call certain star-points, on which increased brilliancy is 

 concentrated, the rest of the retina remaining insensible. The 

 wave-theory of light may assist us here. Our general language on 

 this subject seems based on the emission or corpuscular theory: 

 and allowing this, it is not easy to see how the quantity of light, 

 considered originally as a luminous surface, can ever be increased, 

 not allowing for absorption at the lenses. On the wave principle 

 however, though the very delicate vibrations of ether received 

 through the pupil of the eye may be unable in their simple form 

 to affect the retina, still the much larger quantity of vibrations 

 collected by a large object-glass may have this result. It is true 

 they are spread over a proportionably larger surface of the retina, 

 but being very delicate, they may produce a better effect when 

 " spaced out " than when concentrated : so much so that the light 

 neutralized in passing through the lenses may be more than com- 

 pensated. It may be objected that the case will practically be the 

 same on either theory, and that the rays after passing through the 

 pupil will be as perfectly dispersed as the matter, before reaching 

 the retina : but I think that without theorizing in a random man- 

 ner, it is more conceivable that a stream of light would become per- 

 fectly spread out again after compression, than that separate waves 

 or undulations of light would recover their separate course after 

 comparative confusion in the eye-piece and pupil. When we re- 

 member that the area of the object-glass mentioned above is 720 

 times that of the pupil of the eye, the force of this idea becomes 

 evident. If it is true, as is said by Coddington, that with high 



