246 



arose from the glasses not being at the right distances, there being 

 no means, or the means not having been explained to me, by which 

 the correcting lens could be got at, to shift it. 



4. A very troublesome degree of colour out of the centre of the field. 



This report is of course too meagre and imperfect to conclude much 

 from, but as both Capt. Smyth and Prof. Airy have examined it in 

 much detail, I the less regret that my present circumstances will not 

 allow of my going further into the subject. 



J. F.W. Herschel. 



July 23, 1833. 



Professor Airy's Report. 



From the pressure of business I have had fewer opportunities of trying 

 the telescope than I could have desired. The absence of bright planets 

 also has prevented me from attending so much as I wished to what I 

 regard as the most important point in this construction, namely, the 

 correction of colour. I have, however, had one excellent opportunity 

 of observing the moon, and have observed several stars, single and 

 double, and do not think that my opinion could have been altered 

 by a greater number of observations. The correction of colour is not 

 complete, but it is much more nearly complete than I expected, and 

 very much more so than in a smaller telescope of Mr. Barlow's con- 

 struction which I tried several years since. The colour is so far removed 

 that it is not offensive till a power of 300 is used. But with regard to 

 this colour, there is one point of great importance to be noticed by any 

 person who shall try the telescope in future. It is that, in consequence 

 of the separation of the object lenses, the only part of the field which 

 can possibly be free from colour with a common eyepiece is in the 

 line passing through the centres of the two object lenses j and that 

 from the present imperfect centering, this line falls actually out of the 

 field of the highest power (or quite on the edge). An eyepiece of a 

 different construction and adjusted with greater care is necessary 

 before any positive decision can be given. With regard to the de- 

 finition of a star, it is not at present good, and the telescope is de- 

 cidedly incompetent to separate any close star ; but I regard this as 

 a fault in the making of the surfaces, to which any telescope is liable, 

 and which does not interfere at all in my estimation of the value of 

 the new principle of construction. I know not how far a circumstance 

 mentioned by Mr. Dollond (the alteration of spherical aberration with 

 an alteration of temperature) may account for this ; but so much of 

 the irregularities are cut off by cutting off the external ring of the 

 object glass, that I have no doubt of its being due principally to the 

 figure. 



My opinion is, therefore, that a larger telescope, as good of its kind 

 as the present, would be very useful for nebulse, &c. j and that if freed 

 from defects, which do not appear to belong to the construction, it 

 might be equal to any astronomical work except the examination of 

 bright planets. 



I have had the advantage of trying the telescope once in company 

 with Sir John Herschel, Sir David Brewster, Mr. Cooper, Dr. Ro- 



