REFRACTION. 



todies ctiiefiy, if Mul wlioUy, to tlie fulphureous purts with 

 which they abound. 



From the obfcrvations of M. Euler it appears, that there 

 is no fluid, and probably no tranfparent fubftance of any 

 kind, the refraitive power of which is lefs than that of rain- 

 water, or dillilled water ; and betwixt air and rain-water 

 there is no fubftance that has an intermediate refraftive 

 power. 



After rain-water immediately follows well-water ; but 

 there are probably as many varieties in its refraftive power 

 as there are different wells. We may, however, con/jludc, 

 that the ratio of refraftion from air into well-water is con- 

 tained between the limits of 1.336 to i, and 1.337 to i. 



Spirituous liquors have a greater refraftive power in 

 pi-oportion to their ftrength : but the ratio of refraftion 

 from air into any fpirituous liquor is never lefs than 1.34, 

 nor greater than 1.37. 



There is, probably, no kind of fait, but what, being dif- 

 folved in water, increafes its refraftive power. 



The folutions of fait of urine and of vitriol have the lead 

 refratlive powers, and thofe of rock-falt and fal ammoniac 

 the greateil ; but the ratio of refraftion from air into any 

 faline folution, prcfcrving the proportion of one ounce of 

 fait to twelve of water, will be contained between the limits 

 of 1.34 to I- and 1.35 to i. 



Dillilled vinegar, and the folution of gum arable, have 

 nearly the fame refraftive power with common French 

 wine ; and the refraftive power of white of egg is the fame 

 as that of reftilied fpirit of wine. 



Mineral alkali (aturated, fcems to have the fame re- 

 fraftive power with very fti-ong brandy. 



Spirit of nitre and oil of tartar per deliquium have a 

 medium refraftivc power between fpirituous liquors and 

 oils. 



The rcfraftive powers of oils approach the nearefl to 

 that of glafs, cfpccially oil of turpentine, which had the 

 greateil refraftive power of all the fluids on which he had 

 made experiments. 



Sir Ifaac Newton fufpcfted that different degrees of heat 

 might have fome effeft on the refraftive power of bodies, 

 but his method of determining the general refraftion was 

 not fuffieiently accurate to afcertain this circumftance ; 

 but happily this method of M. Euler's proved to be well 

 adapted to this purpofe. 



From his experiments made for this purpofe, he infers, 

 that the focal diftance of a fingle lens of glafs diminifhes 

 with the heat communicated to it ; and this diminution of 

 the focil diftance is not owing to the increafe of bulk in 

 the glafs by heat ; for the effeft of this change is both in- 

 oonllderable, and of 1 contrarv nature. There can be no 

 doubt, therefore, but that this alteration in the focal diftance 

 is owing to a change in the refraftive power of the glafs 

 itfelf, which, as well as, probably, that of all other tranf- 

 parent fubftances, is increafed by heat, and diminifhed by 

 :cld. 



It may feem furprifing that the focal diftance of a fingle 

 kns fhould decreafe with heat, and yet that of the menif- 

 oufes filled with any fluid, fhould increafe with heat ; but 

 M. Euler obferves, that it by no means follows from hence, 

 that thefe fluids are affefted by heat in a manner different 

 from glafs ; and, after computing the effeft of every cir- 

 cumilance of this complex experiment, of the two glaffes, 

 and the fluid combined, he concludes, that heat increafes the 

 refraftive power of water, and of other fluids, as well as 

 that of glafs. 



He farther obferves, that as 66 degrees of heat diminifhed 

 i',i€ i©cal diilarce j'^lh part, 33 degrees ought to have dimi- 



nifhed it -rjo'l'putj wiitreas kIic diminution in tiiii. cafe 

 was V? t.h. From hence, fays he, one may perhaps conclude, 

 that when it is very cold, tlie fame change in the ther- 

 mometer has a greater effixt on the refraftive power of the 

 glafs than when it is very hot. But he acknowledges that 

 experiments of this kind are not capable of fo much pre- 

 cifion as one could wifh, and that, perhaps in reality, the 

 66 degrees made a change of ^'oth, and the 33 degrees o; 

 -r^cth ; but he imagined that a great number of experiments, 

 made in different timperatures of the air, mi^ht decide this 

 quefUon, cfpecially if objeft-glafles of a very great focal 

 diftance were made ufe of. Ac. Berl. 1762. 



The duke de Chaulnes, not fatisfied with the methods 

 ufed by Newton and others for determining the refraftivc 

 power of glafs, propofed another mode of doing it, which is 

 very ingenious, and, when well condufted, promifing fuc- 

 cefs. He formed the glafs into plates, the furfaces of 

 which were truly plane and parallel, and having placed 

 fmall o!)jefts on each of them, he found, by means of a 

 compound microfcope, to which he applied the molt ex- 

 quifite micrometer, the different diltances at which they were 

 diftinftly vifible, and compared them with the thicknefs of 

 the glafs. This, he fays, gives the proportion of the fines 

 of the angles of incidence and refraftion in that kind of 

 glafs direftly. In this method he afcertained the mean re- 

 fraftive power of 15 kinds of glafs. Ac. Par. 1767. See 

 Priettley's Hift. of Light, &c. p. 483, &c. 



Dr. W. H. WoUafton has propoled a new method of ex- 

 amining refraftivc as well as difperfive powers, by prifmatic 

 refleftion. This method was fuggefted by a conllderatioii 

 of fir Ifaac Newton's prifmatic eye-glafs, the principle of 

 which depends on the refleftion of light at the inner furface 

 of a deiife refrafting medium. 



Since the range of inclination, within which total reflec- 

 tion takes place, depends not only on the denfity of the re- 

 flefting prifm, but alfo on the rarity of the medium adjacent 

 to it, the extent of that range varies with the difference of 

 the denfities of the two media. When, therefore, the re- 

 fraftive power of one medium is known, that of any rarer 

 medium may be learned, by examining at what angle a ray 

 of light will be reflefted from it. 



For inftance, when any objeft is laid under a prifm of 

 flint-glaft, with air alone interpofed, the internal angle of 

 incidence at which the vifual ray begins to be totally re- 

 flefted, and at which the objeft ceafes to be feen by refnic- 

 tion, is about 39° 10' ; but when the objeft has been dipped 

 in water, and brought into contaft with the glafsj -it con- 

 tinues vifible, by means of the higher refraftive power of 

 the water, as far as 57^" of incidence. When any kind of 

 oil, or any refinous cement, is interpofed, this angle is ftiU 

 greater, according to the refraftive power of the medium 

 employed ; and, by cements that refraft more flrongly than 

 the glafs, the objeft may be feen through the prifm^ at 

 whatever angle of incidence it is viewed. 



In examining the refraftive powers of fluids, or of fufible 

 fubftances, the requifite contaft is eafily obtained ; but, 

 with folids, which can in few inftances be made to touch to 

 any great extent, this cannot be effefted without the inter- 

 pofition of fome fluid, or cement, of higher refraftive power 

 than the medium under examination. Since the furfaces of 

 a ftratum fo interpofed are parallel, it will not effeft the 

 total deviation of a ray paffing through it, and may there- 

 fore be employed without rifk of any error in confequence. 



Thus, refin, or oil of faiTafras, interpofpd between plate- 

 glafs and any other prifm, will not alter t'he refult. 



If, on the fame prifm, a piece of felenite and another ,of 



platc-glafs be cemented near each other, their povrers may 



12 bs 



