252 



KNOWLEDGE. 



[November 1, 1900. 



left to those 'who have already gained some exijerience 

 iu this branch of observation, the Leonid meteors being 

 amongst the swiftest that we encounter, since they come 

 to meet the earth, their relative velocity being some 

 forty-four miles per second, the sum, that is, of the 

 actual orbital speed of the meteors, twenty-six miles, 

 and of the eai'th some eighteen miles. The phos- 

 phorescent sti-eaks, due no doubt to this swiftness of 

 motion, which they leave behind them in their path, 

 are. however, a great assistance to the beginner. 



The moon; which reaches its last quarter in the early 

 morning of the I4tli instant, will,"" therefore, interfere 

 very little with the effect of the Leonid shower should 

 we be favoured with a fine one this yeai', and it will 

 have passed to conjunction with the sun before the 

 second great shower of the month, the Andromedes, 

 reach their node. They furnish in all respects a great 

 contrast to the Leonids. The Leonid radiant does not 

 rise on November 14 until 10.30 in the evening; the 

 Andromede radiant is up the entire night, being nearly 

 in the zenith when the Leonid i-adiant is rising. The 

 Leonid meteors are extremely swift; the Andromedes 

 are very slow. The Leonids are distinguished by their 

 green colour, suggesting the presence of magnesium; the 

 Andromedes are rather yellow, as if sodium were their 

 chief constituent. 



The history of the Andromedes is as well known as 

 that of the Leonids. Whilst the latter approach the 

 sun as nearly as the earth's orbit at their perihelion, 

 and recede somewhat bej'oud the orbit of Uranus at 

 aphelion, the Andromedes only recede about half-way 

 between the orbits of Jupiter and Saturn. Their period 

 therefore is one of 6h years as compai-ed with the 33|: 

 of the Leonids, and the greatest showers that we have 

 had from them in recent years have been iu 1872, 1885, 

 and 1892. The shower of November 27, 1872, was one 

 of peculiar interest, inasmuch as it was then clearly 

 recognised that the swarm was moving along the saane 

 orbit which had been travelled by the lost comet of 

 Biela, the comet ■which divided into two portions in 

 December, 1846, and which has never been seen since 

 its return, still in two portions, in 1852. 



Whilst the Leonid shower has been falling gi-adually 

 later and later in the year, so that November 15 is now 

 its date of maximum, the Andromedes, or Bielids as 

 they are indifferently called, have moved from November 

 27 to November 23. This present year is not one m 

 which we have reason to look for the full force of the 

 Andromede shower, but it may well give good practice 

 to the beginner in meteoric observation. The observer 

 should by all means try to record as many paths as 

 possible, the radiant on former occasions having appeared 

 to be rather an elliptic area than as a definite i^oint. 



%ttttvs. 



[The Editors do not hold themselves responsible for the opinions 

 or statements of correspondents.] 



A NEW FORM OF ACHROMATIC TELESCOPE. 



TO THE EDITOES OF KNOWLEDGE. 



Sirs, — A new and original method of correcting cliro- 

 niatism in the combination of lenses suitable for telescope 

 objectives, etc., and for materially reducing the length of 

 such instruments, has been devised and successfully 

 demonstrated by two of the members of the Toronto 

 Astronomical Society ; the Messrs. Collins. 



The method consists essentially of interposing a 



small concavo-convex lens, silvered on the back, in the 

 patli of the converging cone of light that has passed 

 through a large single convex object glass, preferably 

 midway down the cone, both lenses being of the same 

 medium of the same refractive index, the small concavo- 

 ccinvex lens having the requisite curvatures to give an 

 equal amount of dispersion in the opposite direction 

 to that produced by the single object glass. After pene- 

 trating the correcting lens the otherwise scattered points 

 of light are reflected back from the silvered surface, 

 and brought to a focus at a point slightly in advance 

 of the object glass. A total-reflection prism or a flat 

 intercejjts the converging cone before it reaches this 

 point, as in a Newtonian reflector, and throws it to one 

 side, where the image may be examined by an eye-piece, 

 received on a j^hotograjjhic plate, or projected on a 

 screen. 



As the positive dispersion of the object glass and the 

 negative disper.sion of the corrector are of equal and 

 opposite amount, the dispersive effect entirely disappears 

 and an absolutely achromatic image is formed. Spherical 

 abeiTation is controlled by the curvatures of the first 

 objective, and the dimension of field by the size of the 

 prism. A total-reflection prism-lens, of the same re- 

 fractive index as the others, may be substituted for the 

 flat prism, and thereby amplify the focus to any extent 

 that may be required ; the curvatures of the small 

 corrector may be changed to correct for the prism-lens, 

 and the latter in tui-n be made to correct for uniformity 

 of dimensions of all the component colour images, thus 

 delivering to the eye-piece an image focussed entirely on 

 a single plane with component colour images of precisely 

 uniform dimensions on a comparatively wide field. 



The shadow of the flat or prism falls on the concave 

 first surface of the corrector, is reflected back, opens out 

 and covers itself, thus preventing reflections from this 

 surface entering the eye-piece to flare the field. 



Tlie present field -with a 4 in. objective is 4 in. diameter. 

 8 „ „ 5 „ 



Diameter of prism with a 4 „ „ | „ ,, 



„ 8 „ „ 1 „ „ 



Glass of different i-efractive indexes may be used, but 

 when one kind of glass only is employed, the secondary 

 sjjectrum entirely disappears. 



■ \--~Z ^~ 



Collins' Monoplane Achromatic Telescope. 



A sketch of the optical pai-t of the experimental in- 

 strument that has been made is attached herewith. 

 W. B. MussoN, 

 Secretary, Toronto Astronomical Society. 



ARTIFICIAL FACUL^, SPOTS, AND PHOTO- 

 SPHERIC RETICULATION. 



TO THE EDITORS OF KNOWLEDGE. 



Sirs, — If in this letter I take the liberty to address 

 you at some length on the above subject, my one plea 

 must be the articles contributed to your valuable journal 

 at various times by the Rev. A. East. These ai-ticles 

 involve issues which it is impossible to compress into 

 the ordinary sjiace of a letter. 



