202 riJE USE OF THE MICROSCOPr^ 



what more) in the objectives. It has l^econie popuhii', 

 and is now widely employed. 



The Monochromatic Objective. — \'on Rohr began a 

 series of monochromatic objectives by calculating an objec- 

 tive of fused quartz, with an aperture (for a certain ultra- 

 violet wave length) equivalent to 2.5, which was used as 

 a glycerin immersion. It seems probable that the possible 

 uses of this objective have not yet been fully worked out. 

 It is true that it can only be used for photography, and 

 that the ultra-violet light quickly disorganizes living or 

 organic material. (The new monobromide of naphthalin 

 objective, used with blue-violet light, also gives a high 

 aperture, perhaps up to 2.0, for photography.) 



The Monobjective Binocular. — About 1914, Jentsch 

 appUed the method of Abbe's binocular eyepiece, as 

 modified by Ives, with a semitransparent layer of silver 

 instead of a partially reflecting layer of air (Swan cube), 

 to the construction of a binocular microscope. The success 

 of the Greenough had prepared microscopists for a standard 

 binocular for higher powers. This monobjective binocular 

 has also met with success; and it is now made, with varia- 

 tions, by all optical firms. It suffers slightly in some forms, 

 for high-power work, from the fact that changing the 

 interocular distance also changes the optical tube length. 

 However, this could readily be made right. It is used 

 mainly to give a slightly stereoscopic image, not a strongly 

 stereoscopic one. 



Dark-field Condensers. — Jentsch also produced one 

 of the first of the condensers for dark-field illumination, 

 which were used mostly for ascertaining the presence of 

 spirochsetes, but are also found useful for viewdng bacteria 

 and blood parasites. These dark-field condensers give 

 a narrow range of aperture, and usually have two total 

 reflections, first from a convex and then from a concave 

 internal surface (though this is reversed in Nelson's high- 

 aperture form). They have been made with higher and 

 higher apertures, up to 1.27 (Beck); and, in Nelson's 

 Cassegrain and Zeiss's Leuchtbild, even from 1.4 to 1.45, 



