1060 



RECORD OF CURRENT RESEARCHES RELATING TO 



friction. An indicator arm on I marks the degree of inclination on 

 the scale at F. 



If we suppose the apparatus, as figured, to bo in adjustment for 

 central light, then, by turning the milled head D, obliquity of 



incident light is obtained as far as the rack on the sector-bar or the 

 thickness of the stage will permit, the surface of the mirror inclining 

 regularly so that the reflected light is directed constantly in the axis 

 of the condenser throughout the traversing movement. 



The original apparatus from which Fig. 127 was drawn was at once 

 forwarded to us by M. Nachet upon our applying for information on 

 the subject, and at the same time he wrote : " The apparatus was 

 specially designed to keep the focus of the illumination upon the 

 object with varying degrees of oblique incidence. The movement 

 was, however, only from back to front — not lateral. In Grubb's and 

 more modern stands, lateral movement of the substage (unless the 

 lamp be attached to the moving arm) necessitates a continual readjust- 

 ment of the mirror or reflector ; whereas in this device the mirror 

 moves automatically with an exactly calculated differential motion, and 

 the light is constantly directed in the axis of the condenser, conse- 

 quently in the field of view, whatever may be the inclination. The 

 observer can thus watch the minutest changes- developed by the 

 obliquity, which appears to me a considerable advantage." 



Royston-Pigott's Oblique Condenser Apparatus. — Dr. Eoyston- 

 Pigott is the inventor of an appai'atus for giving oscillating oblique 

 action to the condenser. It was thus described * by him : — 



" In former times the precise position of the mirror for throw- 

 ing the rays of reflected light at one particular angle (often hit 

 only with much waste of time and labour) was attained with 

 more or less success so as to give the most brilliant definition of 



* ' Mon. Micr. Jouvn.,' xvi. (1876) p. 178. 



