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On a New Universal Eeflecting Illuminator. 

 By W. K. Bridgman, L.D.S. 



(Read October 27th, 1876.) 

 Plate XIX. (Lower Part). 



Having for several years past been endeavouring to obtain a con- 

 trolling command over the obliquity of an illuminating pencil of 

 light by means of a metallic speculum, placed between the condenser 

 and the object, I was altogether unsuccessful until the advent of the 

 " Bramhall " reflector, which, by suggesting the substitution of a 

 small bull's-eye lens in the place of the ordinary hemispherical 

 condenser, gave a new turn to the direction of the investigation, 

 which has resulted in a complete success. 



The Bramhall arrangement consists of a reflecting surface placed 

 beneath and within a very short distance of the object ; whilst the 

 light, being thrown down as for an opaque substance, first passes 

 through the glass slip, and is then reflected upwards, so as to 

 produce a transparent illumination of great obliquity. There are, 

 however, several very important drawbacks in addition to the diffi- 

 culty of adjustment, and the very limited range of action, which 

 restricts its use to a comparatively small class of objects, and of these, 

 to only a small proportion of such as are mounted in the ordinary 

 way ; yet, all who have once seen the marvellous sharpness and 

 force with which the most difficult diatom tests can be displayed, 

 must be quite ready to admit the principle to be very far in advance 

 of all other modes of illumination, making it highly desirable that it 

 should form the basis of some other special arrangement, that would 

 not only be equally effective, but be easily managed, and free from its 

 other objections, at the same time extending alike superiority of effect 

 to all other classes of objects, and equally applicable under all 

 the common conditions of ordinary mountings — properties that are 

 now claimed for the present arrangement. 



In the illumination of a transparent object there are two considera- 

 tions requiring attention ; first of all, the light should be composed 



