310 



OPTICS. 



Uifi different refrangibilily of llie rays of light. To 

 Descartes is due the honour of first explaining the 

 true theory of the rainbow, and accounting satis- 

 factorily for the formation of the double bow. It is 

 lamentable to think that the brilliant genius of this 

 great man is tarnished with the want of ingenuous- 

 ness ; that the historian of science must have it to 

 tell that in more than one instance he passed over in 

 silence the discoveries of his predecessors, and even 

 condescended to appropriate them as his own. 

 A Unit the same period, or a little later, a remark- 

 able discovery was made by Erasmus Bartholinus, a 

 Dane, who, having obtained some specimens of Ice- 

 land spar, observed the remarkable phenomena of 

 double refraction which they exhibit. In these 

 crystals he remarked that a double image is formed, 

 in one of which the light follows the ordinary law of 

 refraction, while in the other it follows an extraordi- 

 nary law : (an account of his experiments will be 

 found in the 67th No. of the Philosophical Trans- 

 actions.) His experiments were carefully repeated 

 and extended by the illustrious Huygens, who had 

 Uie honour of discovering the polarization of light. 

 To the same philosopher is due the promulgation of 

 the undulatory theory, which was afterwards em- 

 braced by Euler, Dr Young, and most of the conti- 

 nental philosophers, and which has now obtained so 

 much ground among men of science in this country as 

 to threaten the speedy overturn of the Newtonian 

 theory of emission. Huygens employed himself 

 with success in the practical department of optics, 

 and constructed telescopes, some of which were 

 136 feet in length. James Gregory invented the 

 reflecting telescope, which he describes in his Optica 

 Promota, published in 1663, (seeliis life in Cham- 

 bers' Biography) ; but he never lived to see his 

 invention put into operation. This was soon followed 

 by the reflecting telescopes of Newton and Casse- 

 grain. Gregory discovered the law of refraction in 

 the improved form in which we now have it, and 

 made many very accurate experiments on the refrac- 

 tive power of different substances ; so accurate, 

 indeed, were those experiments, that his index for 

 water is not more than 16 ^ 60 in error. About the 

 same time Grimaldi discovered the inflection of light, 

 and Hook made nearly the same discovery, wjthout 

 knowing what had been done by Grimaldi. In the 

 year 1676, the Danish philosopher, Iloemur, from 

 observations on the eclipses of Jupiter's satellites, 

 made the important discovery of the motion of light, 

 and proved that it travels over the diameter of the 

 earth's orbit, a distance of 190,000,000 of miles in 

 eleven minutes of time. Several important contri- 

 butions were about the same time made to optical 

 science by Tachrinhausen and Petit, more particu- 

 larly as regards the investigation of caustic curves, 

 and the structure of the eye. The greatest 

 additions ever yet made to the science of optics 

 were those contributed by the illustrious Newton. 

 In 1664, when in the twenty-fourth year of his age, 

 he commenced that brilliant series of experiments on 

 colours, which led him to the important discovery of 

 the different refrangibility of the rays of light. It 

 would be impossible, in this brief view of the history 

 of optics, to enter even into a catalogue of his dis- 

 coveries in colours and the inflection of light. These 

 are all detailed at full length in his Treatise on Op- 

 tics, one of the finest works on experimental philoso- 

 phy ever composed. The more important of these 

 discoveries will be given in a subsequent section of 

 this article. Although Huygens made experiments 

 illustrative of the polarity of light, yet it was New- 

 ton who first suggested the idea that light had 

 polarity. 



After the discoveries of Newton, the next on re- 



cord, of any importance, is that of the aberration of 

 light, by Dr Bradley. Since the motion of light is 

 such, that a particle of it traverses the diameter of 

 the earth's orbit in eleven minutes, and that, during 

 that time, the earth must have moved through a part 

 of its orbit, it is plain that a ray of light, from a 

 celestial body, will have reached the eye of the 

 observer, after the object and the observer have 

 changed places, from the instant in which the ray 

 was emitted from the object. This is what is called 

 the aberration of light. The invention of the achro- 

 matic telescope has been claimed for Euler and 

 Klingensticorus ; but there can be no doubt that the 

 invention is of British origin, and that it is due to 

 Dolland, who died in 1761, at the age of fifty-five. 

 Euler embraced the undulatory theory of Huygens, 

 with this addition, that he supposed the differences of 

 colour to depend upon the differences of the rapidity 

 of vibrations. Claraut investigated, with great 

 ability, the theory of the achromatic telescope, in 

 which he was followed with great success by d' Alem- 

 bert, and also by the celebrated Boscovich. About 

 1760, Bouguer made numerous experiments, with a 

 view to ascertain the intensity of the light emitted 

 by different objects, and the quantity of light lost 

 by refraction and reflection ; in which interesting in- 

 quiries he was followed by that ingenious German 

 philosopher, Lambert. Sir William Herschel com- 

 menced the construction of reflecting telescopes about 

 the middle of last century, and erected them of greater 

 dimensions than had ever been done before. In 1781, 

 he constructed one of thirty feet in focal length of tho 

 great mirror; and, in 1783, he began the construc- 

 tion of his great telescope of forty feet, the diameter 

 of the great speculum being four feet. Subsequent 

 to this, he investigated the solar prismatic spectrum, 

 and made many important discoveries. About the 

 year 1790, Dr Blair made many valuable experi- 

 ments, with a view to determine the refractive and 

 dispersive power of various substances ; and pro- 

 posed the employment of fluids in the construction 

 of achromatic telescopes. About the same time, 

 Dr Thomas Young began to labour in the advance- 

 ment of optical science. He directed his attention 

 to the mechanism of the eye ; and the results of his 

 observations are highly valuable. He invented the 

 optometer, an instrument for ascertaining the quan- 

 tity of defect in eyes, either long or short-sighted, 

 and, consequently, for determining the exact remedy 

 to be adopted, in order to procure distinct vision. 

 He also applied himself to the phenomena of colours, 

 and of the law of interference ; which last, indeed, 

 he discovered. Dr Young adopted the undulatory 

 theory, and applied it with the utmost perspicuity 

 in the explanation of phenomena. Dr Ritter, of 

 Jena, was the first who directed his attention to the 

 chemical action of the rays of the prismatic spec- 

 trum. Dr Woliaston furnished a paper to the Trans- 

 actions of the Royal society, in 1800, On double 

 Images formed by Atmospheric Refraction ; and, 

 two years afterwards, another, exhibiting A Metliod 

 of Examining Refractive and Dispersive Powers,, 

 by Prismatic Reflection. It was the same philoso- 

 pher who showed that the best form for the glasses of 

 spectacles is the meniscus, with the concave side 

 nearest the eye. He also invented the camera 

 lucida (q. v.). Mains, shortly after his return from 

 the Egyptian campaign, entered vigorously upon the 

 study of optics, and discovered the polarization of 

 light by reflection ; a most important event in the 

 history of experimental philosophy. He found that 

 all bodies polarize light ; but that this is a maximum, 

 at a certain angle, which is different in different bodies. 

 The early death of Malus put a check to his brilliant 

 career, and deprived the. science of one of its bright- 



