766 A TEXT-BOOK OF BOTANY. 



the rays of light to a focus on the objective and against a dark 

 background; and (2) a funnel stop objective. The latter is an 

 ordinary immersion objective with the addition of a funnel stop 

 back of the lenses so that the diffused rays only enter the eye to 

 the exclusion of the direct rays. 



An ordinary microscope with a reflecting condenser and a 

 funnel stop objective thus constitutes an ultra-microscope. The 

 illumination is by means of an arc light. If a Welsbach lamp is 

 used it is necessary to employ a bull's-eye lens to concentrate 

 the light upon the mirror. The light is ordinarily reflected 

 through the condenser from the plane mirror of the microscope. 

 Cover-glasses of a standard thickness, 0.17 mm., should be used. 

 The space between the top of the condenser and the microscopic 

 slide containing the object must be filled with a layer of cedar 

 oil in the same way as between the cover-glass and the objective. 

 Time must be taken to perfectly center the condenser with refer- 

 ence to the objective. 



(Consult: " Dunkelfeldbeleuchtung und Ultramikroskopie," 

 by N. Gaidukov.) 



MICRO-ANALYSIS. 



The value of the microscope is well established in the examina- 

 tion not only of the living plant but in the study of various techni- 

 cal products. It is usual to give greater prominence to the ANA- 

 TOMICAL or HISTOLOGICAL method of study, based largely upon 

 the form of cells and the structure and composition of their walls. 

 The study of cell contents, as starch grains, calcium oxalate, 

 phyto-globulins, and other definite substances, is being utilized 

 very largely in the examination of technical products and to some 

 extent by students of botany. 



A number of books have been published dealing with the 

 micro-chemistry or histo-chemistry of some of these substances. 

 For the most part the study of microscopic crystals has been of 

 a very general nature, in that statements are given regarding 

 the general shape of the crystals or their aggregates and their 

 behavior with certain test solutions. The time has come when 

 the study of the crystalline substances found in plants requires, 

 if any real progress is to be made in this direction, that the 



