916 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Eeagents will often be required to show the structure of animal hairs, 

 which is not so simple as that of those from the vegetable kingdom. 

 The thickness of these hairs makes it desirable that photo-micro- 

 graphs should be made with low-power objectives, as these have the 

 greatest penetrating power. At the same time, good definition is 

 required to show the outlines of the imbricated cells in wool, for 

 example. Wool, ready dyed, of any shade required, is to be had 

 by picking out a little end of coloured worsted. 



Sections of Vegetable Tissues. — Photo-micrographs are especially 

 well adapted as illustrations of vegetable histology ; and the ease 

 with which sections of vegetable tissues are made and mounted for 

 the purpose, as well as the beauty of the result, cannot fail to make 

 this one of the most popular applications of the art. 



Sections — transverse, oblique, or longitudinal — are quickly made 

 with a sharp razor from the petioles of leaves ; from succulent stems, 

 like the new growth of asparagus, geranium, &c. ; from bulbous roots 

 and tubers ; from endogenous plants, such as canna, maize, &c. ; and 

 from recent sprouts on exogenous trees and shrubs. No section-cutter 

 is required for this purpose ; and every one engaged in work of this 

 kind should make himself an expert in free-hand section-cutting, as 

 many of the best photographs are made from extemporaneous pre- 

 parations. 



The proportion of mounted preparations in animal and vegetable 

 histology to be found in every collection which are not suited for 

 photographing, will surprise one who attempts to save himself the 

 trouble of mounting his own specimens for the purpose. 



The first requisite is a very thin section ; the second, a very clean 

 specimen, free from dirt or air-bubbles. To secure cleanliness, wash 

 the leaf or stem or tuber perfectly clean before commencing to make 

 sections, and place the sections in filtered water when they are made. 

 Use a very sharp instrument, and cover the face of the stem, or what- 

 ever it may be, with water or alcohol ; the razor also should be wet 

 before making each cut. 



" Be extravagant in the number of sections cut, and select only 

 the best. The selected sections will often require soaking for a con- 

 siderable time in alcohol, to get rid of the air-bubbles. They are to be 

 mounted in water, solution of acetate of potash, glycerin, or Canada 

 balsam. A little experience will enable the operator to judge 

 whether a section, examined under the Microscope in water, requires 

 a medium of higher refractive index in order to render it more trans- 

 parent. Cells in which the cellulose envelope is comparatively thin, 

 as in the pith of exogenous stems, the epidermis of thin-leaved 

 plants, &c., will show better in water. Thin longitudinal shavings 

 of the wood of the Coniferse — pine, cedar, &c. — may be mounted in 

 glycerin, after being soaked in alcohol to remove air from the cells. 

 Of course water and glycerin may be mixed in any proportion 

 which seems desirable, to secure a refractive index between the two ; 

 and it may be that the addition of chloride of cadmium, or chloral 

 hydrate, to glycerin, for the purpose of obtaining a fluid of still 

 higher refractive index, would in certain cases give still better results." 



