BY A. B. BIGGS. 21 



The only possible amelioration of the obstacle referred to 

 is to leave as much as possible of this disturbing medium 

 below the observer. This is just what is done in the case of 

 the great Lick Telescope, which is erected on a mountain in 

 San Francisco at an elevation of 4,300 feet, or about the 

 height of the top of Mount Wellington. This, coupled with 

 its vast size and perfect finish, places this instrument in the 

 advance post of telescopic power. It has lately been proposed 

 in France to erect an Observatory on one of the Alps, at an 

 elevation of 15,000 feet. Owing to the compressibility of the 

 atmosphere by the superincumbent weight of the upper strata 

 the half of the entire mass lies somewhat below an eleva- 

 tion of three miles, although the rarer air probably 

 extends considerably over 100 miles. But the necessity of 

 air and warmth places a limit not far to reach in this direc- 

 tion. The most daring of balloonists ventured up to an 

 elevation of about seven miles (I speak from memory), when 

 one of the occupants became insensible, and his companion 

 had only strength left to release the gas with his teeth, 

 having lost the use of hands, and descend into a denser 

 atmosphere. 



Assuming that I have fairly stated the case, it would appear 

 then that the smallest advance in telescopic power is neces- 

 sarily to be obtained only at a cost enormously out of pro- 

 portion to the gain, or, as it is commonly expressed, " the 

 game is not worth the candle." 



One word with reference to the announcement of a new 

 optical glass for the manufacture of telescopic lenses, about 

 which a deal of tali talk has been indulged, and probably not 

 a few hoaxes, with more yet to come. This sort of talk 

 springs from an entire ignorance of the function of any such 

 glass and a supposition that some magical magnifying power 

 is inherent in the glass itself instead of in the skill required 

 to shape it. The function of such glass is simply to combine 

 more accurately the different coloured rays, which existing 

 combinations fail, to some extent, to bring to a common 

 focus, the result being more perfect definition. 



A brief explanation will, I think, serve to make this clear. 

 Light passes in straight lines through a uniform medium. On 

 entering a denser transparent body the ray is bent towards 

 the substance of that body (See Fig. 2). 



But what we see as white light is really composed of the 

 blending of all the colours of the rainbow, together with some 

 other rays, viz., the actnic or chemical rays and the heat rays. 

 Now, these latter, together with all the different colours, are 

 unequally "refracted" or bent. The light is, in fact, decom- 

 posed. Of the light rays the violet is the most bent, the red 

 least. They are thus spread out into a band, along which the 

 different colours are distributed. A simple lens, such as that 



