APPARATUS, MATERIAL, AND REAGENTS. 621 



Canada balsam. 

 Dissolved in xylol. 



Glycerine gum (Farrant's solution). 

 Glycerine. 

 Water. 



Saturated solution of arsenious acid. 

 Equal parts ; mix, and add of picked gum arabic half a part. 



Hollis' glue. 

 Zinc-white. 



(E) DRAWING AND PHOTOGRAPHIC APPARATUS. 



Camera Lucida. The camera lucida of Zeiss is an excellent 

 instrument, though many prefer the pattern made by Nachet of 

 Paris. Combined with the use of a micromillimeter objective, it 

 affords also a simple method for the measurement of bacteria. 



For drawing microscopical appearances, and for illustrating 

 microscopical specimens with or without the use of a camera lucida, 

 the following materials should be within reach : 



Pencils. 



Etching pens. 



Prepared Indian ink. 



Water-colour paints and brushes. 



Ordinary and tinted drawing paper and other usual accessories. 



Photo-micrographic Apparatus, Zeiss of Jena, Seibert & 

 Kraft of Wetzlar, Nachet of Paris, and Swift & Son of London, may 

 all be recommended for constructing an arrangement in which the 

 photographic camera is combined with the microscope. 



The best models have been described fully in the chapter on 

 Photography of Bacteria. The accompanying figure (Fig. 249) 

 illustrates a model in which the microscope is used in the vertical 

 position. 



For illumination either sunlight or artificial light may be em- 

 ployed. In the case of sunlight a heliostat is necessary to procure 

 the best results ; but as sunlight is not always available by day, and 

 it is also more convenient for many to work at night, it is better to 

 have recourse altogether to artificial light. Excellent results may 

 be obtained with an ordinary paraffine lamp, or with magnesium, 

 oxycalcium, or electric light. 



