October sth, iSpy.] PROCEEDINGS. ' V 



the protractor B. C is a rod of constant length representing the 

 incident ray and passing through the centre, D, of the protractor 

 and terminating in a pointer C At its other extremity is a 

 projection entering a slot in the slide A. E is a rod divided into 

 parts in terms of C, representing the refracted ray produced, and 

 having the movable projection A' capable of being clamped in 

 any desired position on E. G is a pointer normal to F which 

 represents the refracted ray. H and I are the normals to C and 

 F, the latter being movable. These normals can be adjusted to 

 intersect on the diameter of the protractor, and serve to 

 demonstrate that the refractive index of a substance is simply a 

 measure of the retardation in the velocity of the light in the 

 medium. 



The polarizing angle is shown when the normal G and the 

 incident angle pointers are in the same position on the protractor, 

 but on opposite quadrants. 



The instrument is particularly adapted for solving approxi- 

 mately numerical problems in refraction, as will be seen from the 

 following examples. 



1. If the refractive index of a medium is 1*67, the instrument 

 gives the critical-angle to be 36^°, and angle of polarization 59°. 



2. If the polarizing angle of a medium is 60^, the instrument 

 gives its refractive index 173. 



3. The inner prism of a direct vision spectroscope is required 

 to be of angle 90*^ with a refractive index of i'96, that of the 

 outer ones being 1*52, required the angles of the outer prisms. 



For refraction at the adjacent surfaces put 

 I '96 



= 1*20. 



1-52 ^ 



Set the movable projection to 1*29 and bring refracted-ray 

 pointer to 45'', when the incident-ray pointer shows 66^ or a 

 difference of 2 1 ^ : therefore refraction at first surface must be 

 through this arc (21°). 



Now set the movable projection to 1*52 and bring pointer 

 to show a dif!erence of 21°, we find 31 '9° and 5 2 '9°. 

 90° - 52-9'^ = 37"i° first angle 

 45° + 52*9° = 97*9° second „ 

 45° =45^ third „ , 



