104 THE USE OF THE MICROSCOPE 



exposure, till it becomes solid. Hence only fresh oil from 

 the maker is to be used for immersion. When used as a 

 mounting medium, it hardens slowly on the edges of the 

 cover, but remains liquid, and of the right refractive 

 index, inside. Using thickened cedar oil without a cover- 

 glass, on smears viewed with oil-immersion objectives, 

 introduces an error, unless the objectives are specially 

 corrected for this, or the tube length sufficiently increased. 

 vStains seem to keep well in immersion cedar oil (Bolles 

 Lee, V ademecum) . 



7. Styrax is not often employed for stained preparations . 

 In most preparations of diatoms seen by the writer, this 

 medium is yellow; and it is sometimes cloudy. 



8. Monobromide of naphthalin is used with the Zeiss 

 objective of 1 .6 aperture. The writer employs this objective 

 on thin objects stained with iron-brazilin and mounted in 

 hyrax. Before observation, the hyrax is removed from the 

 spot by xylol. No cover is used. 



9. Hyrax is the trade name of a new high refractive 

 medium. It is said to have a refractive index of about 

 1.8. It shows diatoms well. The writer has found it 

 of use to show fine details on some smear preparations 

 stained with brazilin. It dissolves readily in xylol, and 

 hardens on heating. When objects deep in hyrax are to be 

 viewed with oil-immersion objectives, better images can 

 be had by mixing enough xylol with the drop of immersion 

 oil used. 



10. Realgar is employable apparently only for diatom 

 valves and for rulings on glass. Spitta's excellent photo- 

 graphs of diatoms were a result of his using realgar mounts. 



Permanent and Temporary Preparations. — For the pur- 

 pose of scientific work, we want our preparations to last 

 long enough to be studied, and, if necessary, to be photo- 

 graphed. Whether or not the stain will fade after a few 

 months does not apparently matter, unless the preparations 

 are to be studied by others. However, in the writer's 

 experience, it sometimes happens that the majority of the 

 prepared and stored-up slides are not referred to again. 



