12(J HEltSCHEL ON COLOURED RINGS* 



per angle and distance under them will show the rings con- 

 veniently by transmitted light, while the observer and the 

 apparatus remain in the same situation as if they were to be 

 seen by reflection. 

 3d. Daylight Third Method. A still more eligible way is to use daylight 



reflected up- received upon a plain metalline mirror reflecting it upwards 

 ward from a , , * , , • • , • , • 



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



of the common double microscope ; but 1 forbear entering 

 into a farther detail of this last and most useful way of see- 

 ing rings by transmission, as I shall soon have occasion to 

 say more on the same aubp 

 Generalization. Generalization. Every combination of glasses, that has 

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

 iug 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 arrangements for every desirable purpose of making ex- 

 periments upon rings, as far as single sets of them are 

 Concerned. 



III. Of Shadows. 



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 criti- 

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

 Point of a pen- some slender opaque body will be of eminent service. To 

 teufe cast shadows of a proper size and upon places where they 



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

 eives two si a- When a plain slip of glass or convex lens is laid down, and 

 dews from a the point of a penknife is brought over either of them, it will 

 Vexicliissf >a " cast two sna -dows, one of which may be seen on the hrst sur- 

 face of the glass or lens, and the other on the lowest. 



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

 three from two ,. iii- i 

 glasses, vex lens upon oueshp, so that both are in contact, the pen- 

 knife will give three shadows; but if the convex lens should 

 and in some _ . . „ , .. „ . , ,._.. 

 mmoiom, be of a very short focus, or the slips or glass be a little separa- 

 ted, four of them may be perceived; for in that case there 

 will be one formed on the lowest surface of the incumbent 

 s^lass or lens; but in my distinction of shadows this will not 

 he noticed. Of the three shadows thus formed the second 



will 



