June 19, 1890] 



NATURE 



183 



two objects originally formed one mass, which suffered disrup- 

 tion owing to the vicissitudes encountered in planetary space. 



The average length of path of all the meteors registered is 

 ro°'9. The average height of either fireballs or shooting-stars 

 has been computed, from thirty-eight instances, to be-^ 

 Beginning height ... 71-1 miles. 



End height 48*2 ,, 



From a comparison of a large number of other similar results, 

 the following general average has been deduced : — 



Beginning height ... 76 '4 miles (683 meteors). 

 End height 50-8 ,, (736 „ ). 



If fireballs and shooting-stars are separated, the usual heights 

 of disappearance are : fireballs, 30 miles ; shooting-stars, 54 

 miles. A considerable amount of information as to the radiant- 

 points, stationary and otherwise, has been brought together ; 

 and, with the catalogue, they render Mr. Denning's paper one 

 of a very important character. 



Brooks's Comet {a 1890). — The following ephemeris has 

 been computed by Dr. Bidschof {Aitr. Nach., 2970), and is 

 in continuation of that previously given (vol. xlii. p. 13S). The 

 elements have been found from observations at Cambridge, 

 March 21, and Vienna, April i8 and May 24 : — 



T = 1890 June I '5360 Berlin Mean Time. 



"=68 S4 39"9) 



a = 320 20 32*2 VMean Eq. 1890-0. 

 I = 120 33 5-4 J 

 log q = 0*280524 



EpJumeris for Berlin Midnight. 



Photograph of Brooks's Comet {a 1890). — A photograph 

 of this comet was obtained at Algiers on May 22 by M. Ch. 

 Trepied {Comptes rendus, June 9, No. 23). Two hours' exposure 

 was found necessary. 



ASTRONOMICAL TELESCOPES}- 



B 



EFORE 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 Discourse delivered at the Royal Instituti( 

 by Mr. A. A. Common. 



NO. 1077, VOL. 42] 



1 on Friday, May 30, iSqo, 



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 : after they have passed 

 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 entirely depends on the substance of the lens and the 

 curvature we give to the surfaces, and not at all upon the aperture 

 or width of the lens. The brightness only of the picture of the 

 star, depends upon the size of the lens, as that determines the 

 amount of light it gathers together. If, instead of one star we 

 have three or four stars together, we will find that this lens will 

 deal with the light from each star just as it did with the light of 

 the first one, and just in proportion to the distance they are apart 

 in the sky, so will the pictures we see of them be apart on our 

 screen. So if we let the light from the moon fall on our lens, 

 all the light from the various parts of the moon's surface will act 

 like the separate stars, and produce a picture of the whole moon 

 (in the photographic camera the lens produces in this manner a 

 picture of objects in front of it which we see on the ground 

 glass). When we attempt to get pictures of near objects that 

 do not send rays of light that are parallel we find that as the 

 rays of light from them do not fall on the lens at the same 

 angle to the axis the picture is formed further away from 

 the lens. The nearer the object whose picture we wish to throw 

 upon the screen is to the lens, the further the screen must be 

 moved. If we try this experiment we will find, when we have 

 the object at the same distance as the screen, the picture is then 

 of the same size as the object, and the distance of the screen from 

 the lens is twice that which we have found as the focal length ; 

 on bringing the object still nearer the lens, we find we must 

 move the screen further and further away, until when the object 

 is at the focus the picture is formed at an infinite distance away, 

 or, what is more to our purpose, the rays of light from an object 

 at the focus of a convex lens go away through the lens parallel, 

 exactly as we have seen such parallel rays falling on the glass 

 come to a focus, so that our diagram answers equally well what- 

 ever the direction of the rays ; and this holds good in other cases 

 where we take the effect of reflection as well as refraction. 



We can also produce pictures by means of bright concave 

 surfaces acting by reflection on the light falling upon them. 

 Such a mirror or concave reflecting surface as I have here will 

 behave exactly as the lens, excepting, of course, that it will form 

 the picture in front instead of behind. The bending of the rays 

 in the case of the convex lens is convergent, or towards the axis, 

 for all parallel rays ; if we use the reverse form of lens — that is, 

 one thicker at the edge than in the middle — we find the reverse 

 effect on the parallel rays ; they will now be divergent, or bend 

 away from the axis ; and so with reflecting surfaces if we make 

 the concavity of our mirror less and less, till it ceases and we 

 have a plane, we will get no effect on the parallel rays of 

 light except a change of direction after reflection. If we go 

 heyond this and make the surface convex we then will have 

 practically the same effect on the reflected rays as that given to 

 the refracted ray by the concave glass lens. 



As regards the size of the picture produced by lenses or 

 mirrors of different focal length, the picture is larger just as the 

 focal length is greater, and the angular dimension is converted 

 into a linear one on the screen in due proportion. Now, as we 



