298 Hastings — Aberration for a double Telescope Objective. 



strueted before publishing this paper. The largest objective 

 which could be made of the pieces in my possession was of 2f 

 inches clear aperture. This, though smaller than desired, was 

 sufficient to give a fairly satisfactory answer to the questions. 

 Accordingly the glasses were worked accurately to the curva- 

 tures and thicknesses corresponding to the computations and 

 mounted for use. The astonishing beauty of the images in the 

 new telescope was its most surprising feature at first. The 

 familiar purple was wholly wanting, or at least, could only be 

 recognized with the closest attention, with magnifying powers 

 greater than forty to the inch aperture, and on objects most 

 suitable to its exhibition. But the moment that the instru- 

 ment was applied to astronomical use it was also evident that 

 its defining power was remarkable. The companions to Polaris 

 and Rigel, instead of being objects which require somewhat 

 careful looking, as is the case with my eye and an ordinary 

 achromatic of the same aperture, were strikingly plain. More 

 difficult, but certainly seen, was the fifth star in § Orionis. 

 The binary star yj Orionis was so well elongated that its position 

 angle was estimated to within 5° of its true value ; on the other 

 hand c Ursai Maj. which I suppose to have at present a separa- 

 tion of l //- 7, was divided only with difficulty on a fairly good 

 evening though it was supposed that it would be easy. Saturn 

 showed all that I have seen with an admirable telescope of con- 

 siderably greater apertures, including more than half of Ball's 

 division, the ring C, a single belt and five satellites though 

 Tethys and Dione have not been seen unless they had an elong- 

 ation equal or greater than that of the end of the ring. 

 Rhea has been seen in conjunction. By reference to the rec- 

 ords of many observations which I have made with various 

 telescopes the power of the new telescope was estimated as 

 equivalent to a 3-| inch objective of the ordinary construction. 

 The powers used varied from 53 to 265 diameters with 194 as 

 the most satisfactory for Saturn and for double stars. 



Another method of determining the relative power of the 

 telescope was by comparing the distances at which a table of 

 logarithms could be read with it and a very perfect telescope 

 of 2f inches aperture made a number of years ago, and with 

 which I have observed a great deal. Allowing for the 5 per 

 cent increase in size in the new instrument, the mean of five 

 tolerably accordant determinations indicated a gain of 23 per 

 cent, or that the new objective was equivalent to a 3§ inch ob- 

 jective of the ordinary construction. This ratio of improve- 

 ment is doubtless higher than would generally be admitted as 

 possible by most opticians, but it must stand for the present as 

 the best value attainable. 

 Tale University, March, 1889. 



