158 Notes. 



This was to me a new and highly interesting light upon the 

 subject of the illumination of the microscopic object, and, having 

 the good fortune to fall in with Professor Wright, I mentioned the 

 matter, and Toppler's paper of 1867, to him. He quite shared 

 my interest in Toppler's paper, but told me that the post-objec- 

 tive stop had in fact been made and used, although not ex- 

 tensively. But our discussion of the subject did not end there. 

 We considered how far the generalisation could be carried which 

 enabled the microscopist to choose alternative positions, suitable to 

 the specific requirements of his optical system, for his dark field 

 stop ; and the theory of the matter, when considered from that 

 point of view, very soon yielded the following deductions. 



1. The stop must be placed where the aperture of the instru- 

 ment is largest, when measured by its projection on the image 

 plane of the instrument. 



2. Whether placed in the aperture of the condenser or in 

 that of the objective, the stop will be imaged in the Eamsden circle 

 of the instrument. 



3. If a small opaque disk, having the shape and dimensions of 

 that projected image of the stop, be placed in the exact position 

 occupied by the image, and the original stop be removed, the new 

 disk will shut out exactly the same field light which the original 

 stop shut out in its original position, and so will produce the same 

 dark field in the observer's eye. 



4. Here is a very convenient and very simple apparatus for 

 producing dark field illumination ; for a small stop can easily be 

 mounted in a suitable position over the eye lens and, so placed, 

 will serve to produce dark field with wide-angled lenses. It is 

 obviously unsuitable for narrow-angled lenses, since in that case a 

 stop at the top of the instrument would cut off some — or all, in the 

 case of a full-sized stop — of the light intended to enter the eye. 



These we considered to be conclusions of so great importance 

 as to demand full experimental verification, a task which I under- 

 took, and have now brought to a stage in which I venture to think 

 that the results will be of interest to the Fellowship of the R.M.S. 

 Accordingly, I have now to report as follows : — 



1. A convenient form of stop for use in this way is easily made 

 by breaking up a small mass of mercury into globules, and mount- 

 ing one of these globules on a cover-glass held in the exact plane 

 of the Ramsden circle. For permanent use the globule may be 

 mounted in balsam between two cover-glasses. The precise level 

 is of great importance. Above or below it, the stop appears as a 

 blemish in the optical field, with dark field edges and an opaque 

 centre. Accurately placed in the Ramsden circle, it darkens the 

 whole field evenly and is nowhere opaque. It may be centred or 

 placed excentrically for the purpose of obtaining symmetrical or 

 asymmetrical illumination, as required. 



