75 



On an Apparatus for Facilitating the Use of " Powell's 

 Small Bull's-eye" Illuminator in the Resolution of 

 Test Objects. 



By Geo. Williams. 

 (Read May 24, 1878J 

 Plate VI. 



The attention of microscopists having lately been much devoted 

 to the determination of the merits of object-glasses of recent manu- 

 facture, and to the more or less successful exhibition of beads, lines 

 and stria? on diatoms and other objects, especially with reference to 

 the several condensers and other optical means and appliances by 

 which the desired results can be best obtained, I have thought that 

 a short paper bearing upon the latter portion of the subject might be 

 interesting ; and I beg leave to introduce to the meeting, this even- 

 ing, a contrivance of mine which I have designated " An Apparatus 

 for Facilitating the Use of ' Powell's small Bull's-eye ' Illuminator 

 in the Resolution of Test Objects." 



A very simple, but at the same time highly effective, method of 

 resolving the transverse strice or ribs of Amphipleura pellucida is 

 that employed, and first devised, as I believe, by Messrs. Powell and 

 Lealand. It consists in concentrating an intense beam of light upon 

 the frustule, at a very extreme angle, by means of a small il bull's- 

 eye " of very short focus. For this purpose a super-stage is re- 

 quired, which may be briefly described as a thin platform, slightly 

 larger than an ordinary 3x1 slip, raised upon uprights about an 

 inch above the level of the princij)al stage, the base bearing the up- 

 rights sliding on upon the dovetails which take the upper stage- 

 plate, that having been first removed. The platform, thus fixed, 

 assumes a position perpendicular to the body of the microscope, and 

 has a £" central opening, horseshoe-form, cut out from its side edge 

 to about \ rds of its width ; the light, therefore, on one side of the 

 platform, meets with no obstruction. The slip being laid upon the 



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