166 Transactions of the Royal Microscopical Society. 



ninetieth or the hundredth of an inch. By means of the centering 

 screws, this is brought carefully to the centre of the field of a 

 moderate power. If the illuminating pencil be now carefully 

 manipulated, and the i-^ih. objective put on, it will be found that 

 the image of this aperture can be brought to a focus, presenting a 

 central disk of light with a margin of the black diaphragm as 

 seen in Fig. 1, PI. CXXXI., which presents the aspect of the 

 ''field."* This ai^jjears like centering, and is what we have been 

 usually led to consider, from the methods employed, would he such. 



Now with this very minute aperture in the field take off 

 the -sxjth. objective, and put on a well-corrected \\h. In all pro- 

 bability the disk of light will now be very eccentric, and it must be 

 adjusted again to the centre. This being done, we are led again 

 to suppose that the instrument is centered. But if now the mirror, 

 or still better the rectangular prism be moved to a greater or less 

 angle than that which gives a mere disk, a shaft of light will strike 

 up in the opposite direction ; and by changing the position of the 

 mirror, the direction of this shaft will of course be changed as in 

 Figs. 2 and 3, where the beam a, h is directed from opposite sides 

 of the mirror. This pencil of light is of course at an angle with 

 the plane of the diaphragm, but this effect is not readily produced 

 in a drawing. Now if fortune favours, by some happy movement 

 in the position of the lamp or prism, or both, we may change this 

 beam or shaft of light into an exquisite illumination extending over 

 haK (or even more) of the field, as shown in Fig. 4 ; and we found, 

 in some measure by accident, that this minute aperture could be 

 made to illuminate the whole field. In this condition it presents 

 the appearance of a minute intensely bright sun in the exact centre 

 of the field, with an equal diffusion of rays aU round, except that 

 the intensity of the light grows uniformly less as it reaches the 

 margin of the field. Fig. 5 is scarcely more than a diagrammatic 

 rendering of the effect. 



Now when we had obtained this hght, and used it with the ^g-th 

 and i'd th, we were simply astonished at the beautiful results we ob- 

 tained — results certainly not to be secured by any other means. It 

 opened up structure, and displayed detail in the minute organisms 

 we were studying in a way which gave us hope and pleasure. 



For many of our purposes — such as working out the interior 

 structure of a monad, or studying the changes in a nucleus, or dis- 

 covering the earliest internal evidence of self-division — nothing was 

 equal to this absolutely central illumination, which gave (properly 

 managed) a wealth of light, a delicious field to work with, and 

 simply incomparable results. We found that, however it was to be 

 accounted for, we had fallen upon a fortunate method. But, alas ! 



* Of course it'will be understood that the centering screws will be brought 

 again into requisition to perfectly centre the disk when the g-'^th is put on. 



