AODRKSS. Ixi 



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 centnry ago at the time of Hall and DoUond, namely, the inac- 

 curacy 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 

 cii'cumstance that evaporable liquids being employed between the lenses, 

 a want of permanent imiformity in the instrument was experienced ; and 

 notwithstanding the high degree of perfection to which the grinding and 

 polishing object-glasses has been brought by Clarke, Cooke, and Mertz, not- 

 withstanding the greatly improved instrumental manufacture, the defect to 

 which I have adverted remains imremedied 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. Jacque- 

 lain a few years back ; the last specimen which I have seen is a heavy 

 highly refracting glass formed from the metal thallium byM. Lamy. Among 

 all these could no two or three be selected which, having appropriate re- 

 fracting and dispersing powers, would have the coloured spaces of their 

 respective spectra if not absolutely in the same proportions, at aU events 

 much more nearly so than those of flint and crown glass '? Could not, again, 

 oily or resinous substances, such as castor oil, cauada balsam, &c, having much 

 action on the more refrangible rays of the spectrum, be made use of in com- 

 bination with glass lenses to reduce if not annihilate this signal defect ? This 

 is not a problem to the solution of which there seems any insuperable diffi- 

 culty ; the reason why it has not been solved is, I incline to think, that the 

 great practical opticians have no time at their disposal to devote to long ten- 

 tative experiments and calculations, and on the other hand the theoretic 

 opticians have not the machinery and the skill in manipulation requisite to 

 give the appropriate degree of excellence to the materials with which they 

 experiment ; yet the result is worth labouring for, as, could the defect be 

 remedied, the refracting telescope would make nearly as great an advance 

 upon its present state as the achromatic did on the single lens refractor. 



"While gravitation, physical constitution, and chemical analysis by the 

 spectrum show us that matter has similar characteristics in other worlds than 

 our own, when we pass to the consideration of those other attributes of 

 matter which were at one time supposed to be peculiar kinds of matter itself, 

 or, as they were called, imponderables, but which are now generally, if not 

 universally, recognized as forces or modes of motion, we find the evidence of 

 continuity still stronger. 



When all that was known of magnetism was that a piece of steel rubbed 

 against a particiilar mineral had the power of attracting iron, and, if freely 

 suspended, of arranging itself nearly in a line with the earth's meridian, it 

 seemed an exceptional phenomenon. When it was observed that amber, if 

 rubbed, had the temporary po^^'cr of attracting light bodies, this also seemed 

 something peculiar and anomalous. What are now magnetism and electricity ? 

 forces so universal, so apparently connected with matter as to become two of 

 its invariable attributes, and that to speak of matter not being capable of 

 being affected by these forces would seem almost as extravagant as to speak 

 of matter not being affected by gravitation. 



So -with light, heat, and chemical affinity, not merely is every form of 



