TRANSACTIONS OF THE SECTIONS. 41 



well as chromatic aberration, and at the same time to make the adjacent surfaces 

 fit, very suitable forms were obtained with the data furnished by Mr. Harcourt's 

 glasses. 



After encountering great difficulties from striae, ]\Ii'. Harcourt at last succeeded 

 in preparing- disks of terborate of lead and of a titanic glass which are fairly 

 homogeneous, and with which it is intended to attempt the construction of an 

 actual objective which shall give images free from secondary colour, or nearly so. 



This notice has extended to a greater length than I had intended, but it still 

 gives only a meagre account of a research extending over so many years. It is 

 my intention to draw up a full account for presentation to the scientific world in 

 some other form. I have already said that the grant made to Mr. Harcourt for 

 these researches in 1836 has long since been expended, as was stated in his Report 

 of 1844; but it was his wish, in recognition of that grant, that the first mention 

 of the results he obtained should be made to the British Association ; and I doubt 

 not that the members will receive with satisfaction this mark of consideration, 

 which they will connect with the memory of one to whom the Association as a 

 body is so deeply indebted. 



On one Cause of Trayisparency . By G. Joknsxone Stoney, M.A., F.E.8. 



The motion of the aether which constitutes light is known to be subject to four 

 restrictions : — First, it is periodic ; secondly, it is transversal ; thirdly, it is (at all 

 events temporarily) polarized ; and, fourthly, its periodic time lies between the 

 limits which con-espond to the extent of the visible spectrimi. By temporary 

 polarization is meant the persistence of the same kind of wave over a lon^ series of 

 waves before waves of another kind succeed, that persistence which the phenomena 

 of difiFraction have made known to us *. 



And the many respects in which radiant heat and light have been found to be 

 identical enable us to say that the first three of the foregoing restrictions apply to 

 radiant heat. We also know (see '^Philosophical Magazine' for April and for 

 July 1871) that the lines in the spectra of gases arise from periodic motions in the 

 molecules of the gas, each such motion giving rise to one or more lines corre- 

 sponding to terms of an harmonic series. And we know that under certain con- 

 ditions these lines dilate and run into one anothei-, so as in many cases to produce 

 regions of continuous absorption. All these phenomena may safely be attributed 

 to periodic motions in the molecules of the gas, the dilatation of the lines being 

 due to perturbations which aiiect the periodic times. After the periodic time has 

 been distiu-bed (probably on the occasion of the collisions between molecules) it 

 seems to settle down gradually towards its normal amount,.thus imparting breadth 

 to the corresponding spectral lines. 



The question now naturally presents itself — What results from motions in the 

 molecules which are not periodic, or which are in any other way imfitted to pro- 

 duce radiant heat ? And here the phenomena of acoustics come to our aid. Wnen 

 a bell is struck, more or less regularly, periodic motions are both produced. The 

 more regularly periodic motions produce the tone of the bell which is heard at a 

 distance, while the less regular motions, though they are often very intense, pro- 

 duce a clang heard only in the vicinity of the bell ; in other words, the energy is 

 expended in the neighbourhood of the bell. Similarly, if the molecules of abody 

 are engaged in irregular motions, such motions, though they may occasion a violent 

 agitation of the others, are mechanically incapable of producing such an imdulation 

 as constitutes radiant heat. The disturbance is necessarily local ; in other words, 

 as much energy is restored by the moving sether to the molecules as isimparted by 

 the motion of the molecules to the sether. This absence of radiation is one of the 

 properties of a transparent body ; and the other thermal (or optical) properties of 

 transparent bodies may be presimied to depend also on these partially irregular 

 motions. Thus Fizeau has proved by experiment that a flow of water of about 



* Eays of common light have been found to interfere, of which one was retarded 15 

 millims., or about 30,000 wave-lengths, behind the other, showing what a long series of 

 nearly similar waves usually succeed one another in unpolarized light before waves of 

 another type come in. 



