Royal Astronomical Society. 303 



states, that the mounting and entire arrangements of his 5-feet 

 achromatic are exactly similar to those of Capt. Smyth's larger in- 

 strument, described in the Memoirs, vol. iv. p. 550. The spherical 

 aberration is perfectly destroyed, and the discs of stars shown with 

 beautiful neatness on a dark ground. 



ft Since the middle of the year 1831, I have uniformly placed the 

 stars between the parallel threads in measuring their position. This 

 plan, suggested to me by Sir J. Herschel, I much prefer to any other. 



1 can also speak in the highest terms of the advantage accruing 

 from the use of a red illumination of the field; and I usually employ 

 as deep a colour as the light afforded by the lamp will permit. But 

 the most important improvement I have yet tried consists in the 

 interposition of a concave achromatic lens between the object-glass 

 and its principal focus, by which the focal image being enlarged to 

 above twice its original size, a very high magnifying power may be 

 obtained, while the threads of the micrometer appear of sufficient 

 fineness to permit the measurement in distance of very close and 

 minute stars without distortion. Having frequently felt the incon- 

 venience arising from the threads being magnified in proportion to 

 the power used on the telescope, I stated the difficulty to Mr. Dol- 

 lond, who speedily and most perfectly removed it by the application 

 of the lens above mentioned. The effect is the same, in respect of 

 the power of the telescope and the fineness of the threads, as if the 

 focal length of the object-glass were increased to about 10 feet 



2 inches. 



" The magnifiers, with the micrometer, have been varied accord- 

 ing to the object and the circumstances. At first I usually employed 

 226 j this was subsequently exchanged for 285, which was found 

 more generally efficient ; while 55, 80, and 1 40, were occasionally 

 applied for very faint objects, and 350, 480, 550, and 625, for very 

 close or bright ones. Since the acquisition of the concave achro- 

 matic lens, a power of 295 has been most frequently employed, by 

 using the eye-tube which without it magnifies 140 times; but in 

 very unfavourable circumstances, or on excessively faint objects, 

 170 has been occasionally substituted. The highest magnifier has, 

 however, generally been used, which the state of the atmosphere 

 and the brightness of the object would permit ; and great advan- 

 tage has frequently been derived from the use of 475 or even 600. 



M With the five lower eye-tubes a diagonal prism can be employed, 

 the highest power to which it was originally applicable being 285 ; 

 but since with the second object-lens this eye-tube magnifies 600 

 times, the use of the prism is proportionally extended. And it 

 appears to me that much greater benefit may be derived from this 

 little appendage than merely the increased comfort to the observer, 

 in releasing him frequently from a very awkward position, and 

 placing him in one comparatively luxurious, though this alone exerts 

 no small influence on the correctness of these extremely delicate 

 observations. But I should chiefly recommend the prism to those 

 who are seriously engaged in these researches, as enabling the ob- 

 server to place the stars in any positions he may please with relation 



