1900] MICROSCOPICAL JOURNAL. 269 



the unsatisfactory nature of ordinary colored glasses and 

 fluid cells. The care bestowed on the manufacture of the 

 "Absolutus" eliminates most of the objections, and, in ad- 

 dition, the coloring of the screen is pleasant to the eye, and 

 it does not interfere with the definition of the image. 



The acetylene flame may be rendered monochromatic by 

 the interposition of a screen of cobalt blue glass between 

 the light and substage condenser. 



The principal uses of a light filter in photomicrography 

 are for the correction of the objective, the increase of con- 

 trast in the image, and the increase of resolving power. 

 Dr. S. Czapski has shown that the greatest resolving 

 power is optained by using light or short wave length, 

 even the ultra-voilet. This is due to the fact that the blue 

 end of the spectrum has the shortest wave length, and the 

 limit of resolving power is one-half of the wave-length of 

 the light used. 



Mounting Medium. — In his presidential address to the 

 Quekett Society, Dr. Tatham drew attention to a mount- 

 ing medium consisting of piperine and bromide of anti- 

 mony, with which he has obtained very satisfactory re- 

 sults when examining lined tests. The mixture is prepar- 

 ed by combinning three parts by weight of piperine and 

 two of antimony bromide, by gently fusing the mixture 

 over a spirit lamp, care being taken not to raise the tem- 

 perature more than is necessary or it will char and dis- 

 color. After the diatoms have been spread on the cover- 

 glass in the usual way, a small portion of the mixture is 

 placed between the cover-glass and the slide, and gently 

 fused until a thin film of it unites the two surfaces. When 

 the medium is set it must at once be protected from the 

 air, otherwise the salts will decompose. To effect this, 

 solid paraffin should be allowed to run between the cover- 

 glass and the slide, and the whole finished off with a cir- 

 cle of Hollia's liquid glue. — Knowledge. 



