568 



SCIENCE 



[N. S. Vol. LIV. No. 1406. 



In 187Y Dr. James Edmunds constructed 

 special paraboloid condensers for dark-field 

 work with, high powers, and insisted upon 

 the necessity of a homogeneous contact of 

 the top of the condenser and the lower face 

 of the microscopic slide, and that the slide 

 should have a thickness corresponding to the 

 focus of the condenser. He recommended 

 this means of study for the body fluids like 

 blood, etc., and for the investigation of living 

 bacteria, whose appearance and actions were 

 described b.v him in a most striking and 

 picturesque manner. 



This information was published in some of 

 the most important and widely distributed 

 English publications (Transactions of the 

 Royal Society, 1830, Trans. Micr. 8oc. of 

 London, and Quart. Jour. Micr. Science, 

 1850-1856; Jour. Queh. Micr. Club, and 

 Month. Micr. Jour., 1877; Quekett's "Treat- 

 ise on the Microscope," 1848-1855, and Car- 

 penter's " The Microscope and its Revela- 

 tions," 1856). 



In spite of this wide publicity the dark- 

 field microscope was used very little either 

 in biology or in medicine. After the dis- 

 covery in '1905 of the microbe of syphilis, and 

 that it could be demonstrated in the living 

 state with the dark-field microscope, this 

 method of investigation became of vital im- 

 portance to medical men; and that import- 

 ance has increased rather than diminished 

 in recent years. 



It seems to the writer that it is of equal 

 if not greater importance to the biologist, the 

 physiologist, and the clinician for the ex- 

 amining of the body fluids in health and 

 disease and in the study of living micro- 

 organisms, for it brings out with the great- 

 est clearness structures and details of struc- 

 ture invisible in the bright-field microscope. 

 It thus renders the absolute dependence on 

 staining agents after various fixing materials 

 have been used no longer necessary, and 

 serves as a cheek to the appearances some- 

 times given by these agents. 



Dark-field microscopy has two require- 

 ments that must be met for its successful use 

 as was pointed out by the early investigators 



with it: (1) a very brilliant light is needed. 

 Full sunlight was recommended and remains 

 the most satisfactory light, although the 

 newly devised electric lights like the small 

 arc lamp and the low-voltage head-light 

 lamps serve very well. 



(2) The other difficulty that must be over- 

 come is the large aperture of high-power ob- 

 jectives, especially those of the immersion 

 type. This is because the dark-field conden- 

 sers can not be constructed with high enough 

 aperture to give a dark-field with these high- 

 power objectives, and they are a necessity 

 with the most exacting work. 



Two courses were open with the high 

 powers: (a) To so construct them that the 

 aperture was low enough to give dark-field 

 effects with the dark-field condensers practi- 

 cable to construct, and (b) To introduce into 

 the high apertured objectives a diaphragm 

 that should cut down the aperture. 



The second course was adopted, and reduc- 

 ing diaphragms of all kinds with apertures 

 varying from 0.40 to 0.90 N. A. have been 

 met with; and in a few cases those as low as 

 0.20 N. A. were found. Not only was there 

 great variation in the aperture of the reduc- 

 ing diaphragms for the oil-immersion objec- 

 tives, but in many cases they were so con- 

 structed that they were liable to get out of 

 place, get out of the optic axis, and prove 

 generally unsatisfactory. Unfortunately also 

 some of the workers in the pathological field 

 were trying to use oil-immersion objectives 

 for dark-field work with no diaphragm at 

 all, and of course could get no dark-field 

 effects. 



After a full examination of the different 

 dark-field condensers made in our own country 

 and abroad, it seemed to me that the best 

 all around aperture for the objective to use 

 with them would be about 0.80 N. A. Such 

 an aperture will give a good dark field with 

 all the standard dark-field condensers, and 

 this aperture is great enough to give good 

 resolution on the one hand and the needed 

 brilliancy on the other. 



I appealed to the American manufacturers 

 of microscopic objectives to design and 



