24 INTRODUCTION. 



is made by adding colorless pieces of gum-arabic to a solution 

 of potassic acetate or ammonic acetate, until the liquid becomes 

 of the density of thick syrup, while in that intended for carmin 

 preparations the gum is dissolved in a five to ten per cent 

 aqueous solution of chloral hydrate, and about ten per cent of 

 glycerin added. Either of these media, or a plain solution of 

 pure gum-arabic, will be found to answer admirably for all prepa- 

 rations of woods which are to be photographed. 



115. The edges of the cover-glass are usually painted with 

 some varnish of good quality. Those in best repute are : 



1. Asphalt- varnish, to be thinned with turpentine when too 

 thick. 



2. Maskenlack, a German preparation, thinned with alcohol. 



3. Mikroskopirlack, also thinned with absolute alcohol. 



4. Shell-lac in alcohol, tinged with some anilin color. If a 

 few drops of castor-oil are added to the solution, it dries into 

 a less brittle finish. 



5. Gold-size. 



6. White lead (with oil). 



It is a good plan to revarnish slides whenever the varnish 

 first shows any indication of breaking away. 



A few works in regard to microscopic manipulation and 

 micro-chemistry which may be advantageously consulted by the 

 student are the following : 



BEALE. How to Work with the Microscope (London). This is a large 

 octavo volume, with very minute descriptions of microscopical appliances and 

 manipulation. Several editions have been printed. 



CARPENTER. The Microscope (London). A small octavo of about 900 pp. 

 This work deals at some length with the structure of animals and plants. 



BEHEENS. Hilfsbuch zur Ausfiihrung Mikroskopischer Untereuchungen 

 im Botanischen Laboratorium (Braunschweig, 1883). This is specially de- 

 voted to microscopic manipulation and micro-chemistry. An English trans- 

 lation has appeared. 



POULSEN. Botanical Micro-Chemistry. Translated and enlarged by Pro- 

 fessor Wm. Trelease (Boston, 1884). An excellent account of the chemicals 

 used in the examination of vegetable structures, together with some directions 

 for their employment. 



STRASBURGER. Das botanische Practicum. See an account of this work 

 on page 165. 



BOWER AND VINES. A Course of Practical Instruction in Botany (Ixmdon, 

 1885). A most useful and convenient guide to the study of the histology of 

 flowering plants, ferns, and their allies. 



