THE AMERICAN 



MONTHLY 



MICROSCOPICAL JOURNAL, 



Vol. X. DECEMBER, 1889. No. 12. 



All communications for this Journal, whether relating to business or to editorial 

 matters, and all books, pamphlets, exchanges, etc., should be addressed to Ameri- 

 can Monthly Microscopical Journal, Box 6jo, Washington, D. C. 



European subscriptions may be sent directly to the above address accompa?iiea 

 by International Postal Order for $1.15 per annum, or they may be sent to Messrs. 

 Triibner <§n> Co., 57 Ludgate Hill, London, or to Mr. W. P. Collins, ij? Great 

 Portland street, London, accompanied by the yearly price J five shillings. 



A Dark-Field Stop. 



By WILLIAM LIGHTON. 



LEAVENWORTH, KAN. 



[with frontispiece.] 



Dark-field illumination, when using lenses of high power, and espec- 

 ially homogeneous immersion lenses, has been long desired and is at 

 last accomplished. 



After arranging the mirror so as to obtain central light and removing 

 the eye-piece, on looking down the tube the mirror appears a bright 

 figure in the centre of the back lens of the objective (see Fig. 1, B). 

 Let the large circles (Figs. 1 to 6) represent the back lens of an oil 

 immersion or dry objective of large aperture. On swinging the mirror 

 from right to left its image in the lens will pass from left to right, as 

 indicated in figures 2 and 3. Light from the mirror in this position is 

 known as oblique light. If, when using a dry objective, the mirror is 

 swung so far to the left that its image cannot be taken up by the objec- 

 tive, dark-field illumination is obtained (see Fig. 4, E) . 



When homogeneous-immersion objectives of large numerical aper- 

 ture are used dark-field illumination by the mirror alone is impossible, 

 because such lenses l'eceive light from all points beneath the stage. 



The following method produces a dark field with the mirror in any 

 position from central to extremely oblique : 



A metal frame is used as a carrier for the dark-field stop H (Fig. 7), 

 which is also of metal, and which is joined to the carrier by a fine steel 

 wire, K. The carrier slides in a square nose-piece, L (Fig. 8), be- 

 tween the objective and the body tube of the microscope, as shown at 

 the double-dotted line N (Fig. 8). The nose-piece should have a re- 

 volving fitting, as shown in the sectional view. The handle of sliding 

 carrier is at I (Figs. 7 and 8). The stop H (Fig. 7) mast be of the 



Copyright, 1889, by C. W. Smiley. 



