MICROSCOPIC TECHNIQUE AND REAGENTS. 765 



qualitative and quantitative analytical work. The Spectroscope 

 can be used in checking chemical methods and also employed 

 frequently in the detection of mixtures, just as the microscope is 

 used where qualitative chemical methods are not available. The 

 Spectroscope is used not only in the examination of single color- 

 ing principles, but where there are mixtures, and whether these 

 are in solution, on fabrics, on paper, etc. So that for technical 

 chemists, especially for those interested in dyeing and allied indus- 

 tries, it has a very great value. 



There are several different types of spectroscopes : ( I ) the 

 ordinary, in which the liquid is placed in a long glass cell between 

 the source of light and the slit of the spectroscope; (2) a com- 

 parison spectroscope, where an unknown liquid can be compared 

 with that of a known; (3) the micro-spectroscope, in which a 

 spectroscope is attached to a microscope and the liquid is placed 

 in small tubes. 



A characteristic spectrum is obtained only when the solution 

 is of the proper dilution. The solutions must be prepared care- 

 fully and interfering substances removed as much as possible. 



( Consult : ' Untersuchung und Nachweis organischer Farb- 

 stoffe auf spektroskopischen Wege," by Jaroslav Formanek and 

 Dr. Eugen Grandmougin, Second Edition. " Zur Biologic des 

 Chlorophylls Laubfarbe und Himmelslicht Vergilbung und Etiole- 

 ment," by Ernst Stahl. The Origin and Nature of Color in 

 Plants," Kraemer, in Proc. Am. Phil. Soc., 1904, p. 259.) 



DARK FIELD ILLUMINATION AND THE ULTRA-MICROSCOPE.- 

 The study of minute particles which are otherwise not visible 

 under the microscope by direct illumination may be accomplished 

 by a simple contrivance known as a reflecting condenser. The 

 principle upon which this operates is similar to when a pencil 

 of sunlight enters a more or less darkened room, causing the par- 

 ticles of dust to become visible. In the same manner the invisible 

 particles in a colloidal solution and the ordinarily structureless 

 substances in an animal or vegetable cell are rendered visible by 

 reason of the contrast between these particles and their dark sur- 

 roundings. 



The apparatus consists essentially of two parts: (i) a parab- 

 oloid condenser which has two reflecting surfaces so as to bring 



