166 Transactions of the Society. 



1. The vertical illuminator must not be an oblique, and an 

 oblique illuminator only. 



2. It should be capable of illuminating the full aperture of the 

 back lens of any obiective with a parallel beam of light. 



3. It must not be a permanent attachment to an objective, so as 

 to impair its performance of ordinary work. 



4. The reflector must be placed near the back lens. 



5. There must be some method of regulating the illumination. 

 From the above we may gather that a nose-piece form of 



apparatus is the best, and it would seem that the Beck and Powell 

 forms more nearly conform to the above named conditions, but the 

 reflecting glass must be made much larger than at present, and the 

 hole in the side of the nose-piece should be as large, or nearly as 

 large as the opening of the Society's gauge. 



How to use a Vertical Illuminator to the best advantage. 



This form of illumination can only be used to the best advantage 

 with oil-immersion objectives, and it is the peripheral portions of 

 these objectives which play the most important part with this 

 method. 



Hence we see it is absolutely necessary that the full opening of 

 the Society's gauge should be left perfectly clear, so that the back 

 lens of any objective may have none of its aperture cut down. 



Again, in order that the illuminating source may be focussed 

 upon the object, it is necessary that the path of the light from its 

 .source to the mirror, and from the mirror to the objective, should 

 be equal to that from the eye-piece to the objective. To satisfy 

 this condition with the Eeichert method, the lamp would have to 

 almost touch the body-tube — a quite impossible condition. 



At the hole in the side of the nose-piece there should be a 

 carrier for diaphragms of various sizes — this is preferable to either 

 a wheel of diaphragms or an iris. There should also be a strip of 

 metal with a slit in it, which can be drawn across the hole at the 

 side of the nose-piece. The direction of the slit should be in a line 

 with the edge of the flame of the Microscope lamp. Good illumi- 

 nation can be obtained by carefully attending to the focus of the 

 image of the flame on the object, by placing the lamp at a suitable 

 distance, and by regulating both the size of aperture and the 

 position of the slit. A large cover-glass supplied for compressors 

 would form an excellent mirror ; nothing else is required except the 

 nose-piece adapter with a hole in its side, the diaphragm-carrier, 

 which might with advantage be made capable of rotation, and 

 a few loose stops. As the microscopical examination of metals 

 is now of so much importance, perhaps the few moments spent on 

 the consideration of the vertical illuminator will not have been 

 wasted. 



