MEDIUM-POWER PHOTO-MICROGRAPHY 83 



intricate. But the law underlying the calculations is the same from beginning to end. 

 The same law reversed of course equally holds good when rays pass from the water 

 into air, and when passing from one glass to another, although then with certain 

 modifications. 



One more remark on the subject yet remains to be said. Seeing that n' passing 

 to E becomes refracted to m', what happens to a ray starting about x ? It will pass 

 into the air and graze along E C. If this be true what will take place if one starts 

 still nearer A, say at z ? This ray cannot get out of 

 the water at all and is said to suffer " total reflexion " Fig. 45 



at E, for it appears again at 7!. There is one angle 

 then, it is very evident, which is the last, that allows 

 a ray to get out; this is called the ''critical or 

 " limiting angle',' and is known for all kinds of glass. 



Let us now see how these remarks apply to our 

 subject. Consider Fig. 45. Let C be the cover-glass, 

 having a refractive index of about 1-5, and L the 

 front lens of the objective. Also consider A B an 

 incident ray upon the cover-glass at B. As it enters 

 a denser medium than air, according to our precept 

 it must be refracted towards the normal B N and 

 follow the path shown as B D. When it arrives at D 

 (as in the case of what happens when using a dry lens) it passes into air again — a 

 rarer medium so is bent away from the normal D W as much as it had been bent 

 towards B N the previous normal on entering the glass, thus continuing its course 

 along D E to E ; D E, therefore, is of course parallel to A B. It will now be readily 

 understood, without much consideration, that any rays lying between D E and the 

 edge of the objective (where D E' touches it in the diagram) will be lost to the micro- 

 scope, as they have no chance of entering the front lens of the objective. 



But let us consider what happens if we place between the lens and the cover-glass 

 a drop of fluid, say, cedar oil, which has the same refractive index as the cover-glass 

 itself, viz., 1-5. Follow the diagram, commencing at the right hand, and consider the 

 ray starting from A'. Arriving at B, it will be refracted to D' just the same and for 

 similar reasons as A B was refracted to D. But notice now what happens. As the 

 emerging ray at D' enters a fluid of the same refractive index as the substance it has 

 left, it continues its path uninterruptedly in a straight line to F, which enables the 



