Notes. |159 



2. The size of the stop must be determined with reference to 

 the size of the Eamsden circle, and to the particular result re- 

 quired. I find a range of globules running from ^oo ^ n - ^° 2*0 * n - 

 very convenient, the steps varying by %o6 in. among the smaller 

 members of the series, and t ^q in. among the larger. A stop equal 

 in size to the Eamsden circle gives a black field, upon which only 

 such details are shown as represent highly refractive bodies or 

 objects having a geometrical structure which can be imaged by 

 their own diffracted light. A Pleurodynia angulatum depicted in 

 this latter way, when the central beam is entirely shut out by a top 

 stop covering the whole Eamsden circle, is one of the most beautiful 

 objects that can be presented to the eye. A smaller stop will 

 produce darkening of the field and some degree of strengthening 

 of the definition, and may be very advantageously employed to 

 moderate the glare of excessive illumination. The great advantage 

 of a central stop for this purpose as compared with the common 

 alternatives — as compared, that is to say, with closing down the 

 iris diaphragm or racking down the condenser — arises from the 

 circumstance that moderation of the light is obtained by means 

 of the central stop without any loss of resolving angle, and also 

 with some actual increase of defining power. 



3. Beside the common purposes of producing dark ground, and 

 controlling the illumination to which these top stops can be applied, 

 there is another, which seems to contain the promise of future 

 achievements. By working the iris diaphragm against the stop, it 

 is possible to produce a thin hollow cone of light, which has an 

 extraordinary power of bringing out the stereoscopic detail of an 

 object. Blood corpuscles seen in this way, for example, are no 

 longer fiat rings, but rounded forms with sculptured surfaces. 

 Indeed, it was for this precise effect that Toppler devised his 

 experiment, and few experiments have succeeded more completely. 

 No microscopist will ever forget the first time that he looks under 

 these conditions of illumination into the internal structure of a 

 Triceratium favus mounted dry. 



Mr. Nelson, to whom I have communicated the substance 

 of the above statement, is good enough to furnish me with a 

 reference to this Journal for 1878, page 186, where there is 

 a Note by the late Mr. J. W. Stephenson upon the appearance 

 of P. angulatum when seen under a wide-angled lens with a 

 stop placed immediately behind the objective to shut out the 

 dioptric beam. But Mr. Nelson adds that stops of this kind, 

 although they had a vogue about that time, soon after went out 

 of use. That this should be the case is quite easy to be under- 

 stood. The necessary adjustments cannot possibly be made with 

 a stop placed on the tube of the Microscope; and the actual 

 experiment which Mr. Stephenson demonstrated — of showing an 

 object by its own diffracted light — although extremely pretty, 



