247 



binson, and Professor Hamilton, and their opinion upon the whole 

 coincided nearly with mine. 



I beg to suggest the propriety of attaching a finder to the telescope, 

 as much time is lost in seeking for anv object. 



G: B. Airy. 



June 20, 1833. 



Captain Smyth's Report. 



I beg you will inform His Royal Highness the President, and the 

 Council of the Royal Society, that I have this day packed up the 

 fluid refracting telescope of which they have done me the honour of 

 asking my opinion, and that it will be forwarded to Professor Airy 

 without delay. It might indeed have been sent to Cambridge sooner, 

 but that I waited for the first quarter of the present moon, to test the 

 light and the performance of the instrument ; but I regret that though 

 I was constantly upon the spot, the weather has prevented my having 

 an opportunity of catching her. till she was past her dichotomy, and 

 consequently too glaring for the purpose. 



On the arrival of the telescope, it was carefully unpacked, and im- 

 mediately mounted, for the moment, on the lower slab of the revolving 

 roof of my polar-axis room. It was fitted by its two pivots to the iron 

 cratch which was sent with it, the upper parts of which were cut into 

 Y's : the inner end was supported by Mr. Dolloncl's ingenious eve- 

 end stand." The instrument, however, was liable to tremor, both 

 from the motion of the roof and the floor : but it enabled me to ex- 

 amine a few objects while poles were being prepared to form a better 

 stage outside the observatory. And I should remark, that it was 

 arranged with Professor Airy, who favoured me with a visit on the 

 occasion, that my experiments were to be entirely confined to the 

 performance of the telescope, while he would investigate its principle. 

 My portion was to be governed by direct comparisons with my 

 refractor, as a standard from which to assume the relative merits of 

 the two. That instrument has a double object-glass of dWhs inches 

 clear aperture, and 84- feet focal length ; a space which I have good 

 reason to think is accurately proportioned to the densities of the crown 

 and flint glasses : and notwithstanding the magnitude of the diame- 

 ters, the curves of the lenses seem in tolerably exact chromatic and 

 spherical aberration throughout. It may therefore be presumed to be 

 a more severe reference than the dimensions alone would suggest. 



The temporary stage alluded to, outside the observatory, consists 

 of two upright beams of fir, firmly driven into the bed of gravel which 

 forms the substratum of the garden, and a cross-bar, strongly screwed, 

 supports the iron crutch with its Y's. This is erected close to a plat- 

 form and pier, which were built for some magnetic experiments, and 

 afforded great facility in attending to the outer lens, and augmenting 

 or diminishing its aperture. While looking towards the south, it com- 

 manded from nearly a horizontal view to above 60 c of elevation 5 and 

 by unshipping it, and turning it northwards, it swept the polar region. 

 Such being the means, it remains faithfully to report what I observed, 

 regretting, at the same time, that the weather has continued mostly 

 unfavourable. 



t 2 



