194 CONTINUITY. 



for his most valuable observations and for his exertions in 

 organising a band of observers devoted to the examination of 

 our nearest celestial neighbour ; and to Mr. Nasmyth and 

 Mr. De la Rue for their important graphical and photogra- 

 phical contributions to this subject. The granular character 

 of the sun's surface observed by Mr. Nasmyth in 1860 is also 

 a discovery which ought not to be passed over in silence. 



Before quitting the subject of Astronomy I cannot avoid 

 expressing a feeling of disappointment that the achromatic 

 telescope, which has rendered such notable service to this 

 science, still retains in practice the great defect which was 

 known a century ago, at the time of Hall and Dollond, namely, 

 the inaccuracy of definition arising from what was termed the 

 irrationality of the spectrum, or the incommensurate divisions 

 of the spectra formed by flint and crown glass. 



The beautiful results obtained by Blair have remained 

 inoperative from the circumstance that evaporable liquids 

 being employed between the lenses, a want of permanent 

 uniformity in the instrument was experienced ; and notwith- 

 standing the high degree of perfection to which the grinding 

 and polishing object-glasses has been brought by Clarke, 

 Cooke, and Mertz, notwithstanding the greatly improved 

 instrumental manufacture, the defect to which I have adverted 

 remains unremedied and an eyesore to the observer with the 

 refracting telescope. 



We have now a large variety of different kinds of glass 

 formed from different metallic oxides. A list of many such 

 was given by M. Jacquelain a few years back. The last speci- 

 men which I have seen is a heavy high refracting glass formed 

 from the metal thallium by M. Lamy. Among all these, 

 could not two or three be selected which, having appropriate 

 refracting and dispersing powers, would have the coloured 

 spaces of their respective spectra, if not absolutely in the same 

 proportions, at all events much more nearly so than those of 

 flint and crown glass ? Could not, again, oily or resinous 

 substances having much action on the more refrangible rays 

 of the spectrum, such as castor-oil, Canada-balsam, &c., be 

 made use of in combination with glass lenses to reduce if not 

 annihilate this signal defect ? I have succeeded, to some 



