92 



APPENDAGES TO THE MICROSCOrE. 



Fig. 49. 



surface upon which the Eye-glass is placed. All the reflexions 

 are total, except the first at I ; and the loss of light is far less 

 than would be anticipated. The obliquity which this Prism 

 gives to the visual rays, when the Microscope is placed ver- 

 tically for dissecting or for the examination of objects in fluid, 

 is such as to bring them to the eye at an angle very nearly cor- 

 responding with that at which the Microscope may be most conve- 

 niently used in the inclined position (§ 33, in.); so that, instead 

 of being an objection, it is a real advantage. 



66. Spectroscope Eye-piece. — The researches of Kirchoff and his 



coadjutors having 

 established the value 

 of Spectrum- Ana- 

 lysis* as the most 

 efficient means of 

 distinguishing an im- 

 mense variety of sub- 

 stances — compound 

 as well as simple — 

 when present in very 

 minute quantity, it 

 became obvious that 

 advantage would 

 arise from the appli- 

 cation of the stme 

 method of investiga- 

 tion to Microscopic 

 inquiry ; and, after 

 making trial of dif- 

 ferent arrangements, 

 Messrs. Sorby and 

 Browning (by whom 

 this investigation has 

 been specially pur- 

 sued) have adopted, 

 as the most satisfac- 



Sorby-Browning Spectroscope Eye-piece. 



* By Spectrum-Analysis is meant the study of the Coloured lines or 

 bands characteristic of different substances, when the luminous rays pro- 

 ceeding from them are made to undergo Chromatic Dispersion (§ 11) by 

 passing through a prism or combination of prisms. Thus if a particle 

 of Soda or any of its compounds be heated to incandescence in the slightly 

 luminous flame of a spirit-lamp, the faint spectrum formed by the latter 

 will be crossed by a bright yellow double line ; and whenever this line 

 is observable in the same part of the spectrum, the presence of Sodium 

 may be certainly t inferred. Again, when ordinary luminous rays are 

 made to pass through solutions containing Organic or Inorganic com- 

 pounds, many of these compounds are found to be recognizable by 

 peculiarities in the spectra formed by the rays which they transmit to 

 the prism ; so that, as Prof. Stokes has shown, the colouring matter of 

 Arterial Blood may be in this manner distinguished from that of Venous 

 Blood. (See the Author's "Manual of Physiology," 4th Ed. § 535.J 



