.'Nov. 2 2, 1877] 



NATURE 



67 



the focal length of the eye-piece also increases, and the 

 eye-piece has to be larger, the emergent pencil is then 

 larger than the pupil of the eye and consequently a 



portion of the rays of the cone from each point of the 



object is wasted. 



We get an immense gain to physical astronomy by the 



Fig. 3. — Orion and the neighbouring constellations. 



revelations of the fainter objects which, without the tele- 

 scope, would have remained invisible to us ; but, as we 

 know, as each large telescope has exceeded preceding 

 ones in illummating power, the former bounds of the 

 visible creation have been gradually extended, though 

 even now we cannot be said to have got beyond certain 

 small limits, for there are others beyond the region which 

 the most powerful telescope reveals to us ; though we 

 have got only into the surface we have increased the 

 3.000 or 6,coo stars visible to the naked eye to something 

 like twenty millions. This space-penetrating poArer of 



the telescope, as it is called, depends on the principle that 

 whenever the image formed on the retina is less than 

 sufficient to appear of an appreciable size the light is 

 apparently spread out by a purely physiological action 

 until the image, say of a star, appears of an appreciable 

 diameter, and iSxz effect on the retina of such small points 

 of light is simply proportionate to the amount of light 

 received, whether the eye be assisted bv the telescope or 

 not ; the stars always, except when sufficiently bright to 

 form diffraction rings, appearing of the same size. It 

 therefore happens that as the apertures of telescopes 



i'lo. 4.— The Ncbu'a of Orion, reduced from Lord Rosses Drawing. 



increase, and with them the amount of light (the eye- 

 pieces being sufficiently powerful to cause all the light to 

 enter the e)e), smaller and smaller stars becoire visible, 



while the larger stars appear to get brighter and brighter 

 without increasing in size, the image of the brightest star 

 with the highest power, if we neglect rays and diffraction 



