W. HISLOP ON OBLIQUE ILLUMINATION. 65 



only in outline, and it would not be magnified ; as it is, when we 

 get our rays as nearly parallel as possible, they pass through the 

 slide and the object, enter the objective (where the parallelism 

 ceases), reach the eye through the eyepiece, and the object is seen 

 with more or less distinctness, according to the quality of the 

 objective. But we find, if we turn the mirror sideways from the 

 axis of the instrument, so as to cause the ray of light to impinge on 

 the surface of the slide at an angle, that we may obtain a more 

 distinct view of both the outline and detail of the object; but in so 

 doing, we have introduced a very important change in the condition 

 of the ray of light, — it has become refracted or bent, according to 

 fixed laws, when it touched the first surface of the slide. 



Permit me here to allude to certain phenomena, which are, no 

 doubt, comprehended by members of the Club, but which it is neces- 

 sary to demonstrate in order to make myself clearly understood. 



It is a law of refraction that a ray of light becomes bent when 

 it passes from a medium of a certain density into one of greater 

 density at any other angle than a right angle ; hence a stick 

 plunged into water appears bent at a point coincident with the 

 surface of the water. If it were possible to plunge it into glass, it 

 would appear more bent, because glass is a denser medium than 

 water. The ray of light is refracted towards the perpendicular, 

 when passing from air into water or glass (see Fig. l,pL ^) ; but if 

 the ray pass in the opposite direction — that is, out of a dense into a 

 rarer medium — the bending is away from the perpendicular. If the 

 medium be a plate of glass, with parallel sides, the ray is bent 

 towards the perpendicular when passing through the glass, and 

 away from it and parallel to its original direction when it leaves 

 the glass again. (Fig. 2, pi. A.) 



The result of all this is that our ray of light which we have pro- 

 bably set at a certain angle — say 30 degrees from the perpendicu- 

 lar — passes through the glass at a considerably less angle, and 

 leaves the glass and impinges upon the object at the original angle, 

 provided the object be in the same medium as the original ray — 

 that is to say, provided it be not mounted in balsam or fluid. If 

 the object be mounted in balsam, that medium having nearly the 

 same refractive or bending power as glass, the ray will not impinge 

 upon the object at so great an angle as that of its original direc- 

 tion. The same effect takes place in fluid, but in a reduced 

 degree. 



