212 THE USE OF THE MICROSCOPE 



with a three-quarter.s or .seven-eighths condenser cone, 

 and a small edgeways image of the flame. All his papers will 

 w^ell repay reading, and they usually contain useful practical 

 hints. Hartridge performed some important experiments 

 with the microscope, especially in testing various methods 

 of illumination, including Nelson's method, comparing also 

 opal glass with the direct flame (71). All his papers, mostly 

 in the Journal of the Royal Microscopical Society and the 

 Journal of the Quekett Microscopical Club, are uniquely valu- 

 able. Beck had two papers (29, 32) in the Journal of the Royal 

 Microscopical Society on glare, and these papers are quite 

 important, since he finds the reason for Nelson's use of a small 

 image of the flame edgeways in the field, for critical vision. 

 Conrady in two theoretical papers (48, 49) in the Journal 

 of the Royal Microscopical Society, supplies the reasons on the 

 diffraction theory for the use of a large cone, thus settling 

 a matter which had been long the subject of debate. 



Papers on Dark Field, and Submicroscopic Work. — On 

 dark-field condensers several articles have appeared, among 

 the most important being the following: Siedentopf's 

 papers, on the ultramicroscope (123), on vision of sub- 

 microscopic particles (117), on vision of submicroscopic 

 lines (118), and on dark-field illumination (120) with a new 

 dark-field (cardioid) condenser. Jentzsch had a useful 

 paper (77) on dark-field illumination with a new (con- 

 centric) condenser. Metz (97) calculated an achromatic 

 aplanatic condenser especially for a dark field, in which 

 it gave somewhat better results than the special condensers 

 (paraboloid or concentric) then available. His description 

 is useful to the practical microscopist. Berek (38) 

 explained some of the remarkable color effects produced 

 when stained bacteria are seen on a dark field. Coles 

 (45) found a new way of observing stained spirochsetes, 

 etc., dry, on a dark field, with an immersion condenser. 



Booklets on Color Screens. — With regard to color screens 

 the following are useful: The atlas of absorption spectra 

 by Mees (95), and the booklet (62), probably abstracted 

 from it, on Wratten light filters. These give thorough 



