76j DARK-GROUND ILLUMINATION WITH HIGH POWERS [Cn. II 



i / 36,000 to i / 24,oooth of an inch. They have been called by others free granules 

 or granulations, elementary particles, etc. 



(e) A very striking view of the fibrin network may be obtained by irrigating 

 the thick blood preparation. If a drop of normal salt solution is placed on one 

 edge of the cover-glass and a piece of blotting paper on the other the liquid is 

 drawn through, washing out many of the erythrocytes. If the washing out proc- 

 ess is watched under the microscope the erythrocytes will be seen gliding over 

 or through the fibrin network, or some of them will be anchored at one end and 

 if the current is rapid the corpuscles will be pulled out into pear-shaped forms. 



The leucocytes look like big white boulders in the stream, wholly unmoved by 

 the rushing torrent around them. 



132b. Ultramicroscopy. In ultra- as in dark-field microscopy the objects 

 seem to be self-luminous in a dark space or field. 



Dark- field microscopy deals with relatively large objects, 0.2 fj. or more in dia- 

 meter, that is, those which come within the resolving power of the microscope. 

 Ultramicroscopy on the other hand deals with objects so small that they do not 

 show as objects with details but one infers their presence by the points of light 

 which they deflect into the microscope. This can be made clear by an easily 

 tried naked-eye experiment. Suppose one is in a dark room and a beam of sun- 

 light or electric light is admitted. Unless one is in the path of this beam it will 

 remain invisible, but if there are particles of vapor or dust in the room they will 

 deflect the light and no matter where one stands they will appear as shining par- 

 ticles. The character of the particles cannot be made out, but the light which 

 they deflect enables the observer to infer their presence. 



Dark-field and Ultramicroscopy are said to merge into each other because, in 

 studying specimens like saliva, etc., some of the elements are relatively large and 

 show details, while others are so small that they show simply as points of light 

 with diffraction discs. The larger objects showing details or microscopic resolu- 

 tion, come within the province of the dark-field microscope, while the smallest ones 

 are in the province of the ultramicroscope. With such specimens one might with 

 equal propriety call the instrument used for examining them an ultra microscope 

 or a dark-field microscope depending on whether the attention was directed toward 

 the smallest objects or toward the larger ones. 



COLLATERAL READING FOR CHAPTER II 



BEALE, L. S. How to work with the microscope, pp. 26-29. 



BECK, CONRAD. Cantor lectures, Roy. Soc. Arts, 1907. 



CARPENTER, WM. B. The microscope and its revelations, first edition, 1856. 



Excellent discussion of dark-field microscopy; also in the 6th edition, 1881. 

 CARPENTER-DALLINGER. The microscope and its revelations, 8th edition, 



1901. 

 CHAMOT, E. M. Elementary chemical microscopy. Excellent discussion of 



the dark-field and ultra-microscope. 

 EDMUNDS, JAMES. On a new paraboloid illuminator. Monthly Microscopical 



Journal, Vol. xviii, 1877, pp. 78-85. 

 GAGE, S. H. Dark- field microscopy and the history of its development. Trans. 



Amer. Micr. Soc., Vol. XXXIX, 1920, pp. 95-141- 

 NELSON, E. M. The substage condenser. Jour. Roy. Micr. Soc.; Vol. XI, 



1891, pp. 90-105. 



