Mr. Barrett on the Combination of Rectangular Vibrations. 217 



The explanation of the foregoing facts may be traced to two 

 main causes. The increased transmission noticed with films of 

 the more diathermic liquids chiefly arises from the reduction or 

 abolition of the reflection taking- place from the interior surfaces 

 of the walls of the cell, owing to the optical density of the liquid 

 introduced being nearer to the cell-walls' than that of the medium 

 it replaces. But in glass cells of considerable depth, retaining 

 the former explanation, the augmented heat there observed is 

 probably mainly due to an effect of the refraction of divergent 

 rays by plane surfaces ; this gives rise to a concentration of the 

 beam, which becomes sensible when accompanied by a great 

 transcalency of the liquid in the cell. In similar cells with 

 rock-salt ends the effect is not observed, probably on account 

 of such cells sifting the beam far less than fflass, and thus 

 permitting a higher absorption of the liquid. Nevertheless 

 even with rock-salt cells the causes alluded to must necessarily 

 render, to a certain extent, incorrect the precise absorption 

 hitherto attributed to liquids. These sources of error in de- 

 termining the true absorption by a liquid or solid can, how- 

 ever, be avoided by employing truly parallel rays; and these 

 are best obtained from the sun. 



In conclusion I would add that I do not pretend the observa- 

 tions contained in the foregoing paper can in any material way 

 affect any portion of the celebrated researches on radiant heat 

 which Professor Tyndall has so long conducted. On the con- 

 trary, to my mind, they add lustre to that research, by showing 

 in some degree the difficult and delicate nature of the inquiry. 

 In fact this little investigation of mine is not of such importance 

 as to merit even this reference, were it not that I speak from 

 the sincere and jealous attachment of a disciple to his master. 



XXVIII. On a Simple Method of Exhibiting the Combination 

 of Rectangular Vibrations. By TV. Fletcher Barrett, 

 Lecturer on Physical Science at the London International 

 College, §"c." 



PHYSICISTS are well acquainted with the elegant experi- 

 ments of M. Lissajous, in which the vibrations of two 

 tuning-forks, placed at rectangles, are optically combined by 

 viewing a ray of light successively reflected from a mirror attached 

 to each fork. A regular series of curves is thus obtained which 

 give a perfect optical expression to each of the musical intervals, 

 the curves augmenting in complexity as the dissonance between 

 the forks increases. 



* Communicated by the author, having been read at the British Asso- 

 ciation, August* 1868. 



