Dec. 1 8, 1873] 



NATURE 



129 



An apparatus devised by Prof. Tyndall for experiment- 

 ally demonstrating the laws of reflexion and refraction is 

 admirably adapted for verifying this law. The following 

 description is quoted from his Lectures on Light : — " A 

 shallow circular vessel RIG (Fig. 6) with a glass face, 

 half filled with water rendered barely turbid by the ad- 

 mixture of a little milk or the precipitation of a little 

 mastic, is placed upon its edge with its glass face vertical. 

 By means of a small plane reflector i\I, and through a slit 

 I in the hoop surrounding the vessel, a beam of light is 

 admitted in any required direction." If a little smoke be 

 thrown into the space above the water, the paths of the 

 incident, the reflected, and the refracted beams will all be 

 visible. If then the direction of the incident beam be so 

 adjusted that the reflected and the refracted beams are at 

 right angles to one another, and a Nicol's prism be inter- 

 posed in the path of the incident beam, it will be found 

 that by bringing the vibrations alternately into and per- 

 pendicular to the plane of incidence we shall alternately 

 cut off the reflected and the refracted ray. Thus much 

 for the verification of the law. But not only so, if we take 

 difterent fluids and for each of them in succession adjust 

 the incident beam in the same manner, we shall only have 

 to read off the angle of incidence in order to ascertain the 

 polarising angle of the fluid under examination. 



The polarising angle for glass is 54° 35', water. 



Thus, in Fig. 7, let a, b be the incident, and b, c the 

 reflected ray at the first plate ; b, c the incident, and c, d 

 the reflected ray at the second plate ; then the ray wiU be 

 polarised more or less according to the angle of incidence, 

 at b, and will be analysed at c. 



But in accordance with the principle stated above, 

 viz. that any process which will serve for polarising, will 

 serve also for analysing, we may replace the analysing 

 tourmalin by a second plate of glass (or whatever 

 substance has been used for the first reflexion) placed 

 parallel to the first, and in such a position as to re- 

 ceive the reflected ray ; and if the second plate be then 

 turned round the ray reflected from the first plate b c, 

 as an axis, it will be found that at two positions of rota- 

 tion (first when the plates are parallel and secondly 

 when one of them has been turned through iSo') the 

 light reflected from the second plate is brightest, and at 

 two positions at right angles to the former the reflected 

 ray is least bright. The degree of dimness at the two 

 positions last mentioned will depend upon the accuracy 

 with which the reflecting plates have been adjusted to 

 the polarising angle ; and when this has been completely 

 effected, the light will be altogether extinguished. 



Suppose now that the reflecting substance be, as in the 

 case of glass, transparent. Then it will not be surprising 

 if, when the reflected ray is polarised, the refracted 

 ray should also exhibit traces of polarisation. And in 

 fact every ray of ordinarj' light incident upon a trans- 

 parent plate is partly reflected and partly refracted ; the 

 reflected ray is partially polarised, and so also is the re- 

 fracted ray. This being so,if,instcadof a single plate, we use 

 a series of plates placed one behind the other, each plate 

 will give rise to a series of reflected rays, due to succes- 

 sive internal reflections. The sum of all these will give 

 the intensity and the amount of polarisation of the total 

 reflected light. The phenomenon of these reflexions is 

 therefore rather complicated ; and the modifications due 

 to the additional plates do not materially alter the pro- 

 portion of polarised to unpolarised light. It is, however, 

 otherwise with the refracted rays. The rays transmitted 

 by the first plate enter the second in a state of partial 

 polarisation, and by a second transmission undergo a 

 further degree of polarisation. If this process be con- 

 tinued by having a sufficient number of plates, the ray 

 finally emergent may have any degree of polarisation re- 

 quired.* And it is worthy of remark that, in proportion as 



* Plates of the thinnest description are the best 

 effects, but if the surfaces lie p.irallel and the gl. 

 the number ntay be advantageously increased to i 



)r three give good 

 highly transparent 



the rays become more and more polarised, so does a less 

 and less quantity of light become reflected from the sur- 

 faces of the plates ; and consequently, except in so far as 

 light is absorbed by actual transmission through the 

 substance of the plates, the emergent ray suffers less and 

 less diminution of intensity by each additional plate. So 

 that when a certain number has been attained the inten- 

 sity received by the eye or on a screen is practically unaf- 

 fected by increasing their number. 



Fig. 8 is a general representation of such a pile of pkfes 

 viewed edge-ways. The plates are secured in a brass 

 frame, and the whole supported on a stand. 



W. Spottiswoode 

 [To be continued.) 



yi 



THE ROYAL SOCIETY 

 HE following extracts from the Minutes of the 

 Council of the Royal Society under the dates given, 

 may be interesting to some of our readers : — 



Jan. 26, 1S60. — The President having brought under 

 the consideration of the Council the present scale of re- 

 muneration of the Secretaries, it was resolved :— " That a 

 Committee be appointed to inquire into the matter and 

 report thereon to the Council ; the Committee to consist 

 of the President and the Treasurer, with Mr. Barlow, 

 Mr. Bell, and Dr. Farr." 



Feb. 23, 1 860. — The President presented the follow- 

 ing Report from the Committee appointed on January 26 

 to consider the question of the remuneration of the Secre- 

 taries. 



"Your Committee beg to Report to the Council that, 

 in performing the task which was imposed upon them, 

 they have inquired into the duties of the Secretaries at 

 various times, the gratuities which have been awarded to 

 them, and the financial condition of the Society. 



" They have been favoured with valuable information 

 and opinions by former Officers of the Royal Society, — 

 Sir John Herschel, JMr. Brande, Dr. Roget, and Sir John 

 Lubbock. 



"Previous to the year 1720 no regular salaries were 

 assigned to the Secretaries, but it was customary to 

 present them from time to time with sums varying from 

 10/. to 20/. under the name of Gratuities.' 



" In 1720, on the motion of the President, Sir Isaac 

 Newton, the Council directed that 50/. should be paid to 

 each of the two Secretaries annually. In 1732 this 

 amountwas increased to 60/., and in 1760 to 70/. \os. In 

 November 1799, on the motion of the President, Sir 

 Joseph Banks, the amount of the salaries was recon- 

 sidered by the Council, and raised to One Hundred 

 Guineas to each Secretary, at which amount they have 

 remained from that time to the present. 



"The office of 'Foreign Secretary' originated in a 

 legacy of 500/. bequeathed to the Society in 1719 by Mr. 

 Robert Keck, for the express purpose of remunerating a 

 person for carrying on foreign correspondence. In 1720 

 the first Foreign Secretary was appointed, with a salary 

 of 20/. a year, which sum has been paid, without increase, 

 from that time to the present. 



" It is the concurrent opinion of all who have the best 

 means of knowing, that since the gratuities were last 

 fixed in 1799 the business of the Society and the duties 

 of the Secretaries have largely increased. The increase 

 of Fellows and the larger income of the Society have 

 enabled it to extend its operations. In the ten years 1790 

 to 1799, 319 papers were communicated to the Society ; 

 and in the ten years 1S50 to to 1S59, the number of such 

 papers was 672. Some of the communications are short 

 notices for publication in the Proceedings, and it is im- 

 possible to determine precisely in what ratio the work has 

 increased ; but your Committee are disposed to believe 

 that it is represented approximately by the above figures. 



