442 Report vf the Committee appointed [June, 



Committee think it necessary to mention by way of accounting 

 (and in their minds satisfactorily) for the long interval elapsed 

 from the first reception of the disc to the final completion of the 

 object glass. 



In the state in which it has been submitted unreservedly to 

 their inspection, at Mr. Tulley's house at Ishngton, mounted in 

 a temporary wooden tube, and on a sl.and of very convenient 

 construction for astronomical uses, its clear aperture is six 

 inches and eight tenths, and its performance has proved in the 

 highest degree satisfactory. It has been tried by us on various 

 objects, both by day and by night ; among the latter, the planets 

 Jupiter and Saturn, several of the most delicate and difficult 

 double stars, such as Polaris, y Leonis, ^ Cancri, u^ Leonis, &c. 

 as well as some of the small resolvable nebulas in the constella- 

 tion Virgo ; severe tests these of the performance of a telescope, 

 under magnifying powers from 200 to 700. 



The examination of a bright object on a dark ground, as a 

 card by day-light, or Jupiter by night, with high magnifying 

 powers affords, as is well known, the severest test of the perfect 

 achromaticity of a telescope, by the production of green and 

 purple borders about their edges in the contrary case. The 

 telescope in question bears these tests remarkably well, and 

 is certainly more achromatic than usual, a circumstance de- 

 pending not merely on the nice adjustment of the foci, but on 

 the quality of the flint glass mainly. This might not have been 

 expected (according to a remark of Dr. Brewster) from the 

 high refractive and dispersive power of the glass, but the fact is 

 undoubted. 



The destruction of the aberration of sphericity in an object 

 glass when thoroughly accomplished, even with the best mate- 

 rials, is the strongest proof of the goodness of its workmanship ; 

 but except the materials be good, no excellence of workmanship 

 will destroy that irradiation which surrounds the image of a star 

 with lines of light darting from it as a centre, and which fills the 

 field with loose dispersed hght. The object glass in question is 

 perfectly free from the latter defect, and almost entirely from 

 the former. The rudiments of rays may, indeed, be traced in 

 interruptions of the regular contour of the rings which surround 

 the spurious discs of large stars, and which arise from the inter- 

 ference of the rays grazing the edge of the aperture. Portions 

 of these rings are wanting or very faint, and other portions 

 somewhat stronger ; so that in some directions the outlines of 

 rings of several orders may be traced, — in others only those of 

 the first and second. This defect was distinctly perceived in 

 the image of y Leonis, with a power of 220, giving it the appear- 

 ance of having an excessively faint small star, almost close to 

 the large one of the double star ; but the inconvenience is so 

 slight, that without critical attention, its existence would not be 



