1890.] Mr. A. A. Common on Astronomical Telescopes. 157 



WEEKLY EVENING MEETING, 



Friday, May 30, 1890. 



William Huggins, Esq. D.C.L. LL.D. F.R.S., Vice-President, 



in the Chair. 



A. A. Common, Esq. F.R.S. Treas. R.A.S. M.B.I. 



Astronomical Telescopes. 



Before speaking of the enormous instruments of the present day, 

 with their various forms and complicated machinery, it will be well 

 to give some little time to a consideration of the principles involved 

 in the construction of the telescope, the manner in which it assists 

 the eye to perceive distant objects, and in a brief and general way to 

 the construction and action of the eye as far it affects the use of the 

 telescope, all as a help to consider in which way we may hope to still 

 further increase our sense of vision. 



I will ask you to bear with me when I mention some things that 

 are very well known, but which if brought to mind may render the 

 subject much more easy. Within pretty narrow limits the principles 

 involved in the construction of the telescope are the same whatever 

 form it ultimately assumes. I will take as an illustration the telescope 

 before me, which has served for the finder to a large astronomical 

 telescope, and of which it is really a model. On examination we find 

 that it has, in common with all refracting telescopes, a large lens at 

 one end and several smaller ones at the other ; the number of these 

 small lenses varies according to the purpose for which we use the 

 telescope. Taking out this large lens we find that it is made of two 

 pieces of glass ; but as this has been done for a purpose to be presently 

 explained which does not affect the principle, we will disregard this, 

 and consider it only as a simple convex lens, to the more important 

 properties of which I wish first of all particularly to draw your 

 attention, leaving the construction" of telescopes to be dealt with 

 later on. 



Stated shortly, such a lens has the power of refracting or bending 

 the rays of light that fall upon it : while they are passing through 

 the lens the course they take is altered ; if we allow the light from a 

 star to fall upon the lens, the whole of the parallel rays coming from 

 the star on to the front surface are brought by this bending action to 

 a point at some constant distance behind, and can be seen as a point 

 of light by placing there a flat screen of any kind that will intercept 

 the light. For all distant objects the distance at which the crossing 

 of the rays takes place is the same. It depends entirely on the 



