On the Illumination of Objects under the Microscope. 347 



ciation is necessary for the solution of problems relating to 

 the condition of gases in the atmosphere of the sun and in 

 that of the stars. This seems to be a logical necessity since 

 the electromagnetic theory is far-reaching ; and sensible heat 

 is only one of the manifestations of electrical energy. 

 My conclusions are as follows: — 



1. When a condenser-discharge is sent through a rarefied 

 gas which is confined in a glass vessel, the gas can no longer 

 be considered in the dry state; for aqueous vapour is liberated, 

 from the glass. When a sufficiently powerful condenser- 

 discharge is employed, dry hydrogen, dry nitrogen, and 

 rarefied air give substantially the same characteristic spectrum. 

 When a very powerful steady battery- current is used to 

 excite the tubes filled with these gases, various compounds of 

 nitrogen and oxygen, nitrogen and hydrogen, are formed if 

 aluminium electrodes are employed. 



2. The four-line spectrum of hydrogen in the solar atmo- 

 sphere is an evidence of aqueous vapour, and therefore of 

 oxygen, in the sun. 



3. Conclusions in regard to the temperature of the stars, 

 exhibiting hydrogen spectra, are misleading if based upon 

 conditions of pressure and temperature in glass vessels ; for 

 conditions of electrical dissociation, of aqueous vapour, for 

 instance, in the presence of an excess or lack of supply of 

 oxygen, are the controlling ones rather than conditions of the 

 mere pressure of the gas. 



4. a?-ray phenomena, produced, by a steady battery-current, 

 strongly suggest an electrical theory of the origin of the sun's 

 corona. 



Jefferson Physical Laboratory, 



Harvard University, 



Cambridge, U.S. 



XXX III. An Application of the Method of Striw to the Illu- 

 mination of Objects under the Microscope. By Prof. R. W. 

 Wood *. 



[Plate III.] 



HAVING recently made considerable use of Toepler's 

 " Schlieren-Methode," it occurred to me to see to what 

 extent this most sensitive means of rendering visible minute 

 differences in refractive index could, be applied to the illumi- 

 nation of objects under the microscope. The deflexion of a 

 long ray of light by the stria? in the object to be observed, 

 being the fundamental principle of the method, it seemed at 

 * Communicated by the Physical Society : read June 8th, 1900. 



