78- Experiments for investigating 



Third Method. A still more eligible way is to use daylight 

 received upon a plain metalline mirror reflecting it upwards 

 to the glasses placed over it, as practised in the construction 

 of the common double microscope : but I forbear entering; 

 into a further detail of this last and most useful way of seeing 

 rings by transmission, as I shall soon have occr«iion to say 

 more on the same subject. 



Gejieralization, Every combination of glasses that has 

 been explained in the first, third, and fifth methods of see- 

 ing rings by reflection will also give them by transmission, 

 when exposed to the light in any of the three ways that have 

 now been pointed out. When these are added to the former, 

 it will be allowed that we have an extensive variety of ar- 

 rangements for every desirable purpose of making experiments 

 upon rings, as far as sine;le sets of them are concerned. 

 III. Of Shadows, 



When two or more sets of rings are to be seen, it will re- 

 quire some artificial means, not only to examine them cri- 

 tically, but even to perceive them ; and here the shadow of 

 some slender opaque body will be of eminent service. Ta 

 cast shadows of a proper sise and upon places where they are 

 wanted, a pointed penknife may be used as follows : . 



When a plain slip of glass or convex lens is laid down, 

 and the point of a penknife is brought over either of them, 

 it will cast two shadows, one of which may be seen on the 

 first surface of the glass or lens, and the other on the lowest. 



When two slips of glass are laid upon each other, or a 

 convex lens upon one slip, so that both are in contact, the 

 penknife will give three shadows; but if the convex lens 

 should be of a very short focus, or the slips of glass a little 

 separated, four of them may be perceived; fbr in that case 

 there will be one formed on the lowest surface of the incum- 

 bent glass or lens ; but in my distinction of shadows this 

 will not be noticed. Of the three shadows thus formed the 

 second will be darker than the first, but the third will be 

 taint. When a piece of looking-glass is substituted for the 

 lowest slip the third shadow will be the strongest. 



Three slips of glass in contact, or two slips with a lent 

 upon ihcni, or also a looking-glass, a slip and a' lens put 



together. 



