XIV, 4. Eisen: A successful achromatie light-filter. 445 



One very important feature attaihed with this new light-filter 

 is the differentiation of colors , and the bringing into distinct view 

 various classes of granules which conld not be segregated by day- 

 light alone. 



The natnre of tbe light is a cream-colored background or field, 

 the tissne or transparent object appearing slightly rose-colored 5 i.e., 

 if a double stain is used, such as eosin or congo and methylen-blue. 

 The part colored red is slightly intensified, while the blue of the 

 granules or nuelei is not less distinct. 



For bacteriological purposes the light is valuable, the tubercle- 

 bacilli, for instance, appearing very intensely colored by tbe carbol- 

 fucbsin 011 tbe deep methylen-blue background. 



I have used the light principally for high power work, with 

 Zeiss apochromats of 1*40 aperture, and have thus been able to 

 study my sections with oeulars 12 and 18, witbout any inconvenience, 

 and witbout serious loss of light. When studying the same sections 

 with cloud light, in order to have tbe same amount of light in the 

 field, ocular 8 only could be used. 



Tbe light bas, bowever, two imperfections, which so far I have 

 neitber been able nor particularly anxious to correct. First, it does 

 not bring out properly the blue color of the ordinary hsematoxylin. 

 Second, the color of the liquid used in tbe filter disintegrätes after 

 about twelve bours exposure to intense ligbt. As hsematoxylin is 

 now less used for high power work than formerly, tbe first is not 

 a serious drawback. I know of no way to overcome the second 

 difficulty other than to replenish the liquid each day. If every thing 

 is properly arranged, it takes just three minutes to perform this 

 Operation. The expense is a mere trifle, - - the fraction of a cent. 



Description of the filter. For light-source , I use a 

 Welsbach incandescent gaslight, which can now be had in every 

 city of the United States, and probably everywhere in Europe. 

 YVhere this light can not be had , a coal-oil lamp may be used. 

 The density of the filter as well as its compositum requires a slight 

 modification according to the strength and color of the light. 



For a filter, I use a glass cell, about such as is described 



by Zeiss in bis catalogue of Photographie objeetives, 1897, page 59. 

 This cell should be furnished with an „adapter" by which it can 

 be held in position just below tbe iris-diaphragm in the substage. 

 This brings the cell immediately above the mirror. It should not 

 be placed between the condenser and the iris-diaphragm. as in this 



