SIX LECTURES ON LIGHT. 



they would act in the same manner upon all 

 instruments, the eye included. '1 he identity 

 meant is such as subsists between one color 

 and another, causing them to behave alike ^s 

 regards reflection, refraction, double refrac- 

 tion, and polarization. As regards reflection, 

 \ve may employ the looking-glass used in our 

 first lecture. ^larking any point in the track 

 of the reflected beam, and cutting off the | 

 light by the iodine, on placing the pile at the 

 marked point, the needle immediately starts 

 aside. This is tru? for every position of the 

 mirror. So that both for light and heat the 

 same lav/ of reflection holds good; for both 

 of them also the angular velocity of the re- 

 flected beam is twice that of the reflecting 

 mirror. Receiving the beam on a concave 

 mirror, it is gathered up into a cone of re- 

 flected :ight; marking the apex of the cone, 

 and cutting off the light, a moment's expo- 

 sure of the pile at the marked point produces 

 a violent deflection of the needle. (See Fig. 

 23, where m 111 is the mirror, P the pile, and 

 T the opaque solution.) 



This beam of light now enters a right- 

 angled prism and is reflected at the hvpothe- 

 nuse, i:i a direction perpendicular to its for- 

 mer one. The reflection here is total. Cut- 

 ling off the light, we prove the reflection of 

 the heat to be total also. The formation of 



invisible images by lenses and mirrors may 

 also be demonstrated. Concentrating the 

 beam, and cutting off the light, at the dark 

 focus the carbon-points burn their images 

 through a sheet of black paper. Placing a 

 sheet of platinized platinum at the focus, 

 when the concentration is strong an incan- 

 descent image of the points is immediately 

 stamped upon the platinum. 



And now for polarization and its attendant 

 phenomena. Crossing our two Nicol prisms, 



B, C, Fig. 24, and placing ou pile D be- 

 hind the analyzer, neither heat nor light 

 reaches it; the needle remains undeflect- 

 cd. Introducing the iodine, the slight- 

 est turning of either prism causes the 

 heat to pass, and to announce it- 

 self by the deflection of the needle. Like 

 light, therefore, heat is polarized. Crossing 

 the Nicols ag^in, the heat is intercepted and 



the needle returns to zero. Plunging into 

 the dark space between the prisms our plate 

 of mica, the needle instantly starts off, show- 

 ing that the mica acts upon the heat as it did; 

 upon the light : we have in both cases the 

 same resolution and recompounding of vi- 

 brations. Removing the mica, the needle 

 falls to zero ; but, on introducing a plate of 

 quartz between the prisms, the consequent; 

 deflection declares the circular polarization of 

 the heat. For double refraction it is neces- 

 sary that our images should not be too large- 

 and diluted : here are the two disks pro- 

 duced by the splitting of the beam in Ice- 

 land spar. Marking the positions of the disks, 

 and cutting off the light, the pile finds in its 

 places two heat-images. The needle now- 

 stands near 90, and, on turning the spar,, 

 the deflection remains constant. Transfer- 

 ring the pile to the other image, the cU-fiec-. 

 lion of 90 is maintained ; but on turning 

 the spar the needle now falls to zero. The 

 reason is manifest. Permitting the light to- 

 pass, we find the luminous disk at some dis- 

 tance from the pile. We are dealing, in 

 fact, with the extraordinary *bcam which 

 rotates round the ordinary So that fr.~ > t-at 

 as well as for light we have double refraction, 

 and also an ordinary and extraordinary ray. 

 (In the adjacent figure, which shows the ex- 

 perimental arrangement, N is the nozzle cf 



