70 COSMOS, 



cording to the ingenious explanation of my friend, high mag- 

 nifying powers facilitate the discovery and recognition of the 



champ du telescope sera d'autaut plus tranchee qu'on fera usage d'un 

 grossissement plus fort." 



" The eye is endowed with ouly a limited sensibility ; for when the 

 light which strikes the retina is not sufficiently strong, the eye is not 

 sensible of any impression. In consequence of deficient intensity, many 

 stai's escape our observation, even in the darkest nights. Telescopic 

 glasses have the effect of augmenting the intensity of the images of the 

 stars. The cylindrical peucil of parallel rays emanating from a star, 

 and striking the surface of the object-glass, on whose circular surface it 

 rests as on abase, is considerably contracted on emerging from the eye- 

 piece. The diameter of the first cylinder is to that of the second as 

 the focal distance of the object-glass is to the focal distance of the eye- 

 piece, or as the diameter of the object-glass is to the diameter of the 

 part of the eye-piece covered by the emerging rays. The intensities 

 of the light in these two cylinders (the incident and emerging cylin- 

 ders) must be to one another as the superficies of their bases. Thus, 

 the emerging light will be more condensed, more intense, than the nat- 

 ural light falling on the object-glass, in the ratio of the surface of this 

 object-glass to the circular surface of the base of this emerging pencil. 

 As the emerging pencil is narrower in a magnifying instrument than the 

 cylindrical pencil falling on the object-glass, it is evident that the pupil, 

 whatever may be its aperture, will receive more rays, by the interven- 

 tion of the telescope, than it could without. The intensity of the light 

 of the stars will, therefore, always be augmented when seen through a 

 telescope. 



" The most favorable condition for the use of a telescope is undoubt 

 edly that in which the eye receives the whole of the emerging rays, 

 and, consequently, when the diameter of the pencil is less than that of 

 the pupil. The whole of the light received by the object-glass then co- 

 operates, through the agency of the telescope, in the formation of the 

 image. In natural vision, on the contrary, a portion only of this light 

 is rendered available, namely, the small. portion which enters the pupil 

 naturally from the incident pencil. The intensity of the telescopic im 

 age of a star is, therefore, to the intensity of the image seen with the 

 naked eye, as the surface of the object-glass is to that of the pvpil. 



" The preceding observations relate to the visibility of one point or 

 one star. We will now pass on to the consideration of an object having 

 sensible angular dimensions, as, for instance, a planet. Under the most 

 favorable conditions of vision, that is to say, when the pupil receives 

 the whole of the emerging pencil, the intensity of each point of the plan- 

 et's image may be calculated by the proportions we have already given. 

 The total quantity of light contributing to form the whole of the image, 

 as seen by the naked eye, will, therefore, be to the total quantity of tho 

 light forming the image of the planet by the aid of a telescope, as tiie 

 surface of the pupil is to the surface of the object-glass. The compar- 

 ative intensities, not of mere isolated points, but ofthe images of a plan- 

 et formed respectively on the retina of the naked eye, and by the in- 

 tervention of a telescope, must evidently diminish proportionally to the 

 superficial extent of these two images. The linear dimensions of the 

 two images are to one another as the diameter of the object-glass is to 

 that of the emerging pencil. We therefore obtain the number of times 

 that the surface of the magnified image exceeds the surface of the im* 



