THE AMERICAN 



MONTHLY 



MICROSCOPICAL JOURNAL. 



VoL.X. DECEMBER, 1889. No. 12. 



Ali communications for this Journal, ivheiher relating to busitiess or to editorial 

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

 can Monthly Microscopical Journal, Box 6jo, lVashi7igto7i, D. C. 



European subscriptions 7nay be sent directly to the above address accompaniea 

 by International Postal Order for S^-^5 per antium, or they may be sent to Messrs. 

 Triibner &^ Co., j/ Ludgate Hill, Lofidon, or to Mr. W. P. Collins, 157 Great 

 Portland street, Londoji, accompanied by the yearly price of five shillings. 



A Dark-Field Stop. 



By WILLIAM LIGHTON. 



LEAVENWORTH, KAN. 



[with frontispikce.] 



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. i, B). 

 Let the large circles (Figs, i 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 lai-ge numerical apci"- 

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

 because such lenses receive light from all points beneath the stage. 



The following method produces a dark field with the mirror in an\ 

 position from central to extremelv 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) must be of the 



Copyright, 1889, by C. W Smiley. 



