582 



SUMMAKY OF CURRENT RESEARCHES RELATING TO 



their hypothenuse surfaces contiguous so as to form a parallel 

 plate, are placed in front of a telescope having an object-glass at J, 

 with crossed threads in its focus at F, and a second (eye-piece) lens 

 at L. Suppose that any fluid, or semi-fluid, of less refractive index 

 than the prisms, is spread in a thin layer between them, and they are 

 rotated on an axis at right angles to the plane of the paper. When 

 the telescope, with the prisms in front of its objective, is now directed 

 to any bright object, a given point P of the latter will send a pencil 

 of parallel rays through the prisms in a direction parallel to the 

 axis of the telescope. These rays will therefore be collected by 

 the objective J to the centre of the ocular-field, at F, and the eye 

 behind the (xjular will see that point of the field illuminated. This, 

 however, will only be so as long as the angle y, under which the parallel 

 rays reach the internal surface of the prisms, is less than the critical 

 angle corresponding to the difference in the refractive indices of the 

 flint and the fluid between the prisms. If, according to the position 



Fig. 113. 



of the prisms, this angle should exceed the critical angle for the 

 rays which are directed to F, no ray can reach this point of the 

 ocular field, nor any other point F' of the lower half of the field (as 

 the diagram is drawn). For the rays which could be collected by 

 the objective J to such a point F', must of necessity enter the 

 objective as a parallel pencil inclined upwards in front of J, and (as 

 will be readily seen by considering the refraction of the prisms) must 

 meet the surface C under greater obliquity than the axial pencil just 

 considered ; consequently they will undergo total reflection if the 

 axial pencils are totally reflected. Under the condition assumed 

 above, the points of the upper half of the field only could possibly 

 receive light through the prisms, because the parallel pencil which 

 is directed to such a point (F") is inclined downwards, and is there- 

 fore transmitted through the surface C under a smaller obliquity. If 

 now, by rotating the prisms, the angle y for the rays directed to the 

 central point F of the field, ahouhl just be equal to the critical angle, 

 all points of the field above F will receive light through the prisms, 

 whilst all points below F will remain dark, provided monochromatic 

 light is used; the observer will therefore see through the ocular 

 one half of the field bright and the other half dark, the intersect- 

 ing line of both halves just coinciding with the crossed threads 

 at F. By noting the angle a at which this occurs the refractive 



index of the fluid is readily obtained. For sin ^ = - sin a (t? being 



the refractive index of the glass prisms) y = ft -{- iv and n = v Bin y. 



