TRANSACTIONS OF THE SECTIONS. 27 



phosphoric acid, while raising, it is true, the dispersive power,_ at the same time 

 produces a separation of the colours at the blue as compared with that at the red 

 end of the spectrum, which ordinarily beloupfS only to glasses of a much higher 

 dispersive power. A telescope made ot disks of glass prepared by Mr. Ilarcourt 

 was, after his death, constructed for Mrs. Harcourt by Mr. Iloward Grubb, and was 

 exliibited to tlie Mathematical Section at the late Meeting in Belfast. This telescope, 

 which is briefly described in the ' Report' *, was found fully to answer the expec- 

 tations that had been formed of it as to destruction of secondary dispersion. 



Several considerations seemed to make it probable that the substitution of titanic 

 acid for a portion of the silica in an ordinary crown glass would have an effect 

 similar to what had been observed in the phosphatic series of glasses. Phosphatic 

 glasses are too soft for convenient employment in optical instruments ; but should 

 titano-silicic glasses prove to be to silicic what titano-phosphatic glasses had been 

 foimd to be to phosphatic, it would be possible, without encountering any extra- 

 vagant curvatures, to construct perfectly achromatic combinations out of glasses 

 having the hardness and permanence of silicic glasses; in fact the chief obstacle at 

 present existing to the perfection of the achromatic telescope would be removed, 

 though naturally not without some increase to the cost of the instrument. But it 

 would be beyond the resources of the laboratory to work with silicic glasses on 

 such a scale as to obtain them free from striae, or even sufhciently free to permit of 

 a trustworthy determination of such a delicate matter as the irrationality of 

 dispersion. 



When the subject was brought to the notice of Mr. Hopkinson he warmly 

 entered into the investigation ; and, thanks to the liberality with which the means 

 of conducting the experiment were placed at his disposal by Messrs. Chance 

 iJrothers, of Birmingham, the question may perhaps be considered settled. After 

 some preliminary trials, a pot of glass free from strins was prepared of titanate of 

 potash mixed with the ordinary ingredients of a crown glass. As the object of 

 the experiment was merely to determine, in the first instance, whether titanic acid 

 did or did not confer on the glass the unusual property of separating the colours at 

 the blue end of the spectrum materially more, and at the red end materially less^ 

 than corresponds to a similar dispersive power in ordinary glasses, it was not thought 

 necessary to employ pure titanic acid ; and rutile fused with carbonate of potash 

 was used as titanate of potash. The glass contained about 7 per cent, of rutile ; 

 and as rutile is mainly titanic acid, and none was lost, the percentage of titanic 

 acid cannot have been much less. The glass was naturally greenish, from iron 

 contained in the rutile; but this did not affect the observations, and the quantity 

 of iron would be too minute sensibly to affect the irrationality. 



Out of this glass two prisms were cut. One of these was examined as to 

 irrationality by Professor Stokes, by his method of compensating prisms, the other 

 by Mr. Hopkinson, by accurate mea'sm-es of the refractive indices for several definite 

 points in the spectrum. These two perfectly distinct methods led to the same 

 result — namely, that the glass spaces out the more as compared with the less 

 refrangible part of the spectrum no more than an ordinary glass of similar disper- 

 sive power. As in the phosphatic series, the titanium reveals its presence by a 

 considerable increase of dispersive power ; but, unlike wliat was observed in that 

 series, it produces no sensible effect on the irrationality. The hopes, therefore, that 

 had been entertained of its utility in silicic glasses prepared for optical purposes 

 appear doomed to disappointment. 



P.S. — Mr. Augustus Vernon Ilarcourt has now completed an analytical deter- 

 mination which he kindly undertook of the titanic acid. From 2-171 grammes of 

 the glass he obtained -13 gramme of pure titanic acid, which is as nearly as possible 

 per cent. 



On tlie Effects of Heat on the Molecular Structure of Steel Wires and Jiods. 

 By Professor W. F. Baeuetx. 



* Keport for 1874, Transactions of the Beetious, p. 26i 



3* 



