PHOTOMICKOSCOPE : J. H. G. MONYPENNY 145 



prevalence of unsuitable material for the disc may account for the 

 opinion frequently held that for such low powers the prism illuminator 

 gives the better effect. A prism would give a much better result 

 man Fig. 5. 



There are three other points in connection with the vertical 

 illuminator that the author would like to mention. 



(1) The illuminator is rarely made large enough to fill the back 

 lenses of the lower power objectives — for example, the 24 mm. 

 N.A. .30 or the 12 mm. N.A. .65. The only disc illuminator known 

 to the author which is large enough for these lenses is the large 

 pattern made by Watsons, London. 



(2) The illuminator should be fitted with an iris diaphragm, 

 which should have some type cf centering adjustment. ihis iris 

 is used in the same manner as the iris on a substage condenser, and 

 should therefore close absolutely central with the objective. Such 

 adjusting movements as are found on the Watson pattern mentioned 

 above are suitable. 



In connection with the prism illuminator it is curious that in 

 the pattern as ordinarily sold, the iris diaphragm closes concentrically 

 with the middle of the front face of the prism, and therefore with 

 a line about one-eighth of an inch from the centre line of the 

 objective. The iris should, of course, close concentrically with the 

 middle of the bottom edge of the prism, as shown in Fig. 6, where 

 A indicates the centre line of the iris as ordinarily fitted, and B 

 the line on which it should close. The effect, on the performance of 

 an objective of short focal length, of closing the iris about line A 

 can be imagined. 



(3) One of the great defects of the disc illuminator, especially 

 with the lower power objectives, is the presence of flare due to 

 the reflection of the incident light by the outer surface of the back 

 combination. This is a matter, however, which could probably be 

 remedied to a great extent by the objective designer. It will be 

 obvious that (other things being equal), the more convex this back 

 surface is, the less the amount of flare, since more of the reflected 



light will ibe reflected on to the inner surface of the draw tube 

 (and be absorbed by the blackened surface), and less will reach the 

 eye-piece. The author has one lens in his possession in which the 

 back surface is apparently slightly concave, and, owing to the 

 amount of flare caused thereby, the lens, though a magnificent one 

 from every other point of view (it is the Zeiss 12 mm. Apochromat 

 N.A. .65) is not so valuable metallographically. The author would 

 suggest that this is a point to which opticians could usefully give 

 their attention in computing objectives for metallurgical work. 



It is, of course, obvious that with the present method of con- 

 struction of objectives there is much more likelihood of flare being 

 obtained with apochromatic objectives than with achromatic — 

 especially with the lower powers. In the former the back com- 

 bination has very little magnifying power, its function being chiefly 

 that of correcting the aberrations and other faults of the front 

 combinations. In the simpler achromatic the back combination 

 frequently has a considerable magnifying power. The more convex 

 back surface of the latter type of lens will therefore cause less 

 flare than the less convex surface of the more highly corrected 

 combination. The author has frequently noticed this difference in 



