QQ W. HISLOP ON OBLIQUE ILLUMINATION. 



But each case of bending or refraction has to a certain extent 

 decomposed the ray of light;— it is no longer in the same state ; 

 dispersion has taken place, and colour has been produced which 

 will often puzzle the observer, leading him to blame his object 

 glass or his condenser, unless he is quite conversant with the dif- 

 ferent effects of chromatic aberration in these two combinations 

 for widely different purposes. 



But there is yet a phenomenon to be noticed, which is also in- 

 variable in its effects. If we incline our mirror still .further from 

 the axis of the microscope, we shall find that we shall arrive at a 

 point when the light no longer passes through the slide, is no 

 longer transmitted, but reflected ; and at a certain angle for differ- 

 ent media, this becomes total reflexion. The angle of total reflex- 

 ion for water is given at 96°, and for glass at 83°. This very 

 interesting and beautiful phenomenon, which almost seems to 

 shew the materiaUty of light, forms the principle of construction 

 of some of the best optical appliances used in the microscope. In 

 the right angled prism, for instance, which makes the best reflector 

 known (Fig. S,pl. 4j, we have a figure of three sides, two of which 

 are at right angles, or 90° with each other, and the third at an 

 angle of 45° with the other two. A ray of fight impinging per- 

 pendicularly upon the upper surface, passes through without 

 change, but striking upon the inner surface dividing the glass 

 from another medium of less density (namely, the air), it becomes 

 reflected (not refracted), and passes out of the other face of the 

 prism unbent, unaltered and therefore achromatic. 



Take again the glass reflector used for the table polariscope. At a 

 certain angle a portion of the rays having passed through the glass 

 and impinged upon a non-reflecting surface, are absorbed, and the 

 remainder are reflected in a polarised condition from the surface, 

 if we observe at what is known as the polarising angle, which is 

 for glass 56° 45'. If we increase this angle, total reflexion ensues, 

 and the ray, resuming its original condition, is merely bent or re- 

 flected from the surface of the glass. 



It follows from these considerations that there is a practical 

 limit to the angle at which rays of light can be transmitted from 

 one medium into another. In other words, we cannot pass rays of 

 light through a plate of glass so as to impinge upon an object on the 

 upper surface of that glass beyond a certain angle. If we employ 

 such means as lenses to modify our illumination, we shall have 



