WRIGHT: AN IMPROVED VERTICAL-ILLUMINATOR 



15 



high power objectives it is less satisfactory because the reflecting 

 prism cuts off half of the rays from the objective and thus seri- 

 ously impairs the resolving power and general efficiency of the 

 optical system.^ The second type of illumination with the Beck 

 illuminator does not suffer from this defect as the light which is 

 reflected from the thin glass plate passes, on its return after 

 reflection from the metal surface, thru the glass plate itself on its 

 way to the eye of the observer. The glass plate thus serves both 

 to reflect and to transmit the light; 

 the resulting intensity of illumination 

 is, however, noticeably less than that 

 obtained by the first method. For 

 satisfactory work the glass plate 

 should be plane and thin and the 

 source of light so arranged that none 

 of the rays reflected from the objec- 

 tive lens surfaces reach the eye of the 

 observer, otherwise they cast a haze 

 or fog over the entire field, thus re- 

 ducing the contrasts and flooding the 

 image with false light. 



The device illustrated in figure 1 

 was constructed in the Geophysical 

 Laboratory five or six years ago, to 

 correct this defect and has been found 

 so satisfactory and useful in practice that it is perhaps worthy 

 of brief description. It enables the observer to produce an aper- 

 ture of any desired size in any part of the field ; he has thus con- 

 trol over the entire field and can eliminate any incident rays 

 which would otherwise disturb the kind of illumination desired. 

 He can moreover obtain rays of any desired obliquity of inci- 

 dence and thus increase or decrease the apparent relief of the 

 surface under observation. 



The adjustable aperture is obtained by fitting to the ordinary 

 vertical illuminator (Bausch and Lomb type) four cylindrical 



Fig. 1. 



Improved vertical 

 illuminator 



1 Carl Benedicks, Metallurgie 6: 1-4. 1904. 



