165 



V. — On the Vertical Illuminator. 

 By Edwakd M. Nelson. 



(Read Feb. 17th, 1904.) 



With reference to the Vertical Illuminator, which, after lying 

 more or less in abeyance for some twenty-five years, has lately 

 come into notice for the examination of opaque objects, a few 

 words may be said both on its construction and use. 



There are four forms of this appliance now catalogued by 

 opticians. 



1. That known as Tolles' interior illuminator. This consists of 

 a minute prism let into the side of the objective mount, either just 

 behind the duplex or triplex front, or the back lens. 



This form may be criticised, because (a) it gives, and can only 

 give, oblique illumination ; (/3) it requires a special modification of 

 the objective mount ; (7) the objective cannot perform so well as it 

 ought to do when it has a portion of its aperture at its periphery 

 permanently stopped out. 



(Zeiss' form of Tolles' illuminator blocks out half the objective 

 aperture.) 



2. The Beck form : this consists of a nose-piece containing a 

 cover-glass for a reflector, mounted so as to be capable of rotation. 



This has the advantage (a) that the objective aperture is left 

 quite clear ; (/3) that in the objective itself there is neither the 

 modification nor the structural alteration that the Tolles' method 

 requires. 



This device may be criticised because (a) only a portion of the 

 back lens can be illuminated by a parallel beam when the circular 

 cover-glass is rotated to an angle of 45°, and (/3) it has no means of 

 regulating the illumination. 



3. Powell's form, like Beck's, consists of a nose-piece, but it 

 contains a piece of worked glass fixed permanently at an angle of 45° 

 in place of the rotating cover-glass. It has, however, an attached 

 wheel of diaphragms for the purpose of regulating the light. The 

 criticism relating to this device is that the opening is so small that 

 (a) it cuts down the aperture of the objective ; (/3) the whole of 

 the back lens cannot be illuminated by a parallel beam. 



4. Iteichert's form is somewhat similar to Powell's, only it is 

 mounted in an eye-piece adapter at the upper end of the draw tube, 

 and it has no light regulator. The criticism upon this device is 

 (a\ the reflector is too far from the back lens of the objective, and 

 (/3) there is no means of regulating the illumination. 



By casting one's eye over the criticisms on these forms of 

 vertical illuminators, it is quite easy to see what is, and what is 

 not wanted. 



