1869.] Dr. Robinson on the Great Melbourne Telescope. 315 



I. "Appendix to the Description of the Great Melbourne Telescope. - " 

 ByT.R.Roi?iNsox,D.D.,F.R.S.,&e. Received February 10, 1869. 

 (Abstract.) 



Since this paper was read the author has made several observations of 

 the quantity of light transmitted by object-glasses, and determined the 

 index of absorption in various specimens of glass. The results of some of 

 these are in accordance with the opinion expressed in the paper ; but 

 others present a difference which is very satisfactory as indicating a sur- 

 prising progress in the manufacture of optical glass. The observations 

 were made by means of Zollner's photometer. 



The following results were obtained for the intensity of the light trans- 

 mitted by a variety of object-glasses : — 



Description. 



Aperture. 



Focus. 



Intensity. 





in. 



in. 







2-75 



48 



0-5497 





3-80 



63 



0-5962 





3-25 



48 



0-6567 





6-50 



96 



0-6772 





5-50 





0-7928 





500 





0-8739 





1200 



224 



0-8408 



h. Double 



3-20 





0-7393 



Of the above, a belongs to the Armagh Observatory ; it is by one of the 

 Dollonds, older than 1790, and is probably one of their first attempts at a 

 triple combination, b is the original object-glass of the Armagh circle ; it 

 was made by Tulley about 1828 ; the crown is greenish, and is supposed 

 to be English ; the flint is believed to have been from Daguet. c was made 

 for the author by Tulley in 1838 ; its glass is French, the crown is 

 greenish, d is by Cauchoix ; the crown is greenish, and has probably a 

 high n, but its mean thickness is only 0*39. e is by Messrs. Cooke ; the 

 glass is Chance's, /is by Grubb, the glass Chance's : the very high trans- 

 mission of this lens is in part due to the cementing of the adjacent sur- 

 faces, which, while it makes more difficult the correction of spherical 

 aberration, removes almost entirely the reflection at a surface of crown and 

 one of flint : the factor for this =0'9036 ; and if the I be multiplied by 

 this, we obtain 07806, nearly that of e, the difference being due to the 

 reflection at the film of cement, g is also by Grubb, and cemented ; the 

 glass is by Chance, h is by Fraunhofer. 



On examining this Table the progressive increase in the light of the 

 object-glasses is evident. The first two, which may be considered good 

 specimens of the early achromatics, have less illuminating-power than the 

 Herschelian reflector. A great advance was made. by Guinand and those 

 who followed in his steps ; and a still greater one by Chance, whose glass 

 is nearly perfect as to colour and transparency. 



The same inference follows from the author's measure of the index of 



2 A 2 



