8 



Transactions of the Society. 



Fig. 8. 



the globule, we then have the conditions, illustrated by fig. 3, under 

 which .Fresnel rings are formed, by the turning back upon itself of 

 a small annular wave-front reflected from a narrow zone lying 

 about the equator on the surface of the globule. Here Z is a 

 section of the reflecting zone. A is a section of the ring upon 

 which an incident annular wave front B C is focused, and A' is a 

 section of the ring to which it is reflected. B D is a section of a 

 surface all parts of which lie at equal optical dis- 

 tances from the ring A'. Under these conditions 

 the illumination at A' will be a maximum, if the 



3 v 5 \ 



distance C D = 0, or — — , etc., and will be a 



2 2 



minimum, if it is equal to X, 2 X, 3 X, etc. Thus, 

 taking A' in a series of different positions rela- 

 tively to A, we get varying illumination, which 

 passes from maximum to minimum and back to 

 maximum successively, with the result of a 

 system of Fresnel rings. 



Under these conditions very magnificent dis- 

 plays of Fresnel rings can be produced, especially 

 if the field is darkened by means of a top stop, 

 and they have, in a way presently to be men- 

 tioned, an important application for the purpose 

 of testing and perfecting the centring adjustments of the sub- 

 stage apparatus. 



Eeturning now to the consideration of the bright spot in the 

 middle of the globule, the first thing that strikes the observer is 

 that a very large quantity of light is there reflected, and that the 

 object on the stage is in fact receiving a very powerful top light from 

 the objective. This impression, upon examination, proves to be well 

 founded. In Plate I. fig. 2 we have a photograph of a small piece of 

 etched tinfoil. This object was illuminated entirely by light thrown 

 back from the refracting surfaces of the Microscope. If metallur- 

 gists wanted only to examine minute pieces of metal like this 

 fragment, which, in fact, measures about y^ inch in diameter, 

 they would not have need to have recourse to any other system of 

 illumination than that which is furnished by reflection from the 

 lenses of the objective. This fact has an important bearing upon 

 the appearance of all small objects seen in the Microscope. To 

 this cause, for example, are due some of the most striking appear- 

 ances presented by diatoms. And it is now apparent to me that to 

 this cause must be attributed the high light shown upon the 

 specimen of Staphylococcus, a photograph of which I showed to 

 the Society in November last. The photograph is reproduced in 

 Plate III. of the Journal of the Society for 1907 (facing p. 10). A 

 very familiar illustration of this effect of top lighting is presented 

 by the well-known appearance of Pleurosigma angulation under a 



