PROCEEDINGS OF THE ROTAL SOCIETY. 



679 



earned by the friction of the upper wings against each 

 other, which appears to be a mode of conveying intelli- 

 gence between the sexes, indicates that these insects must 

 be provided with organs of hearing; They are incapable of 

 flying, but their wings assist them in swimming. 



On the 24th of June, two communications from Wil- 

 liam Hyde WoUaston, M.D. F.R.S. were read. The first 

 was on a method of examining refractive and dispersive 

 powers, by prismatic reflection. It was suggested to the 

 author by a consideration of the prismatic speculum em- 

 ployed by Sir Isaac Newton in his reflecting telescope. 

 The angle, at which the total reflection of light of any kind 

 first takes place at the surface of a rarer medium, depends 

 on the comparative density of the two mediums in contact; 

 and hence the measurement of this angle readily furnishes a 

 determination of the ratio of refraction at the common sur- 

 face, for the kind of light observed. Thus, by means of a 

 triangular prism, a drop of each of two or more fluids be- 

 ing placed side by side on the under surface, it may easily 

 be found, by inclining the prism more and more, which 



■ of the dark spots first disappears, and It follows that the 

 respective fluid has the weakest refractive power. But 

 when a solid is examined, it must in general be united by 

 the interposition of some fluid of a higher refractive den- 

 sity, otherwise the contact will be too imperfect ; and it is 

 ft'.sily shown that this interposition does not affect the ulti- 

 mate result. But for determining at once the numerical ratio 

 of the sines. Dr. Wollaston has invented an apparatus, in 

 which,by means of a rectangular prismof flint glass, the index 

 of refraction of each substance is read off at once by a ver- 

 nier, the three sides of a moveable triangle performing the 

 operations of reduction of the ratios in a very compendious 

 manner. In this method it is obviously unnecessary that 

 the substances to be examined should be of any determi- 



■ nate ferm ; and it s as easy to ascertain the refractive den- 

 sity of the most opaque as of the most transj>arent bodies, 



■ provided they be less refractive than the pttsm 'employed. 

 It may also serve as a chemical test, for example in essen- 

 tial oils, which when adulterated are generally rendered less 

 refractive ; and a very minute quantity is sufiicient for the 

 experiment. Where the medium is of variable density, 

 this is almost the only mode in which its refractive power 

 can be ascertained ; hence it is of singular utility in exa- 

 mining the refraction of the crystalline lens. (Phil. Trans. 

 1801.41.) A copious table of the refractive powers of 

 various substances is here inserted. The dispersive powers 

 ot different substances are inferred from similar observa- 

 tions upon tlie fringes which usually accompany, or rather 



constitute, the boundary of reflection : the author observes 

 that they are sometimes wanting, or even reversed, when 

 the dispersion is equal at different angles of deviation, or 

 when it is greater even with a less deviation, as when oil 

 of sassafras is applied to a prism of flint glass, as well as in 

 many cases of spars with fluids. Solutions of metallic salts 

 in general are found to be very highly dispersive : by weak- 

 ening the solution till the line of separation became colour- 

 less, and then noting the refractive density. Dr. Wollaston 

 has been able to compare the dispersive powers of several 

 such substances with that of plate glass. He has also ar- 

 ranged a number of substances in a table, in the order of 

 their dispersive powers, at a given deviation ; an order ma- 

 terially different from that of their refractive density. A 

 very important observation concludes this part of the essay. 

 Dr. Wollaston observes, that by looking tljrough a prism at 

 a distant crevice in a window shutter, the division of tire 

 spectrum may be seen more distinctly than by any other 

 method, and thai the colours are then only four; red, 

 yellowish green, blue, and violet, in the linear proportions 

 of the numbers IS, 23, 36, 25 ; and that these proportions 

 will be the same whatever refractive substance be em- 

 ployed, provided that the inclination of the prism remain 

 unchanged. In the light of the lower part of a candle, the 

 spectrum is distinguished by dark spaces into five distinct 

 portions. 



Tlie second paper was On the oblique refraction of Ice- 

 land crystal. It contains a confirmation of the experiments 

 of Huygens on this substance, with ailditional evidence, 

 deduced from the superiority of Dr. WoUaston's mode of 

 examining the powers of refraction. He observes, that Ds. 

 Young has already applied the Huygcnian theory with con- 

 siderable success to the explanation of several other optical 

 phenonaena, and that it appears to be strongly supported 

 by such acoincidcnceof thecalculationideilucedfrom it,wiih 

 the results of these experiments, as could scarcely have 

 happened to a faUc theory. Huygens supposes the undu- 

 lations of light to be propagated in Iceland crystjl in a 

 spheroidal instead of a spherical form ; and infers that the 

 ratio of the sine of incidence to the oblique ordinate of re- 

 fraction mjist be constant in any one section, but different 

 for different planes. Dr. Wollaston observes, that, though 

 we do not fully understand the existence of a double 

 refraction, and are utterly at a loss to account for the phe- 

 nomena occurring upon a second refraction, by another piece 

 of the spar, yet that the ^jlique refraction, when considered 

 alone, is nearly as weH' explained as any other optical phs- 

 nomenon. 



