THE OPTICAL EQUIPMENT 43 



unsightly, and, as they usually have a broad diffraction-fringe 

 round them, become exaggerated in appearance. On the other 

 hand, a discreet use of the method of increasing magnification 

 by using a high-power eye-piece may be at times of great 

 service. An example of this will perhaps best indicate the 

 point in view. 



If an ordinary one-sixth objective, with an aperture of O75, 

 is being used with a No. 8 ocular, this is probably about the 

 highest eye-piece that such a combination permits without 

 appreciable loss of definition. If for this is substituted an 

 8-mm. apochromatic objective, which has approximately 

 double the focal length of the other lens, and which has an 

 aperture of at least 0*65, it may be used with an ocular of more 

 than double the power without suffering in any way, and there 

 would not be a substantial difference in resolution between the 

 two arrangements. But there would be a very appreciable 

 advantage when using the longer focus lens ; considerably more 

 depth of focus would be obtained, and the correction of the 

 image in general would be of a very much higher order. It 

 must not be overlooked that nearly all one-sixth-inch ordinary 

 objectives are actually of shorter focus than that so that 

 their actual aperture is, in reality, lower relatively than it 

 appears to be. 



This question of the best combination of objective and 

 ocular is one that must be largely determined by the conditions 

 under which the work is being carried out, and would be 

 influenced by the class of object that is being dealt with. 



Mention must be made of an ingenious suggestion of 

 Mr. J. W. Gordon, 1 by which the narrow beams of light 

 transmitted by high-power eye-pieces are so altered that the 

 beam becomes a wide one. Further, by his method the black 

 specks, due to dust or any imperfection of any description in 

 the apparatus, or those due to imperfections of the fluid of the 

 eye, are not apparent. A description of this method is here 

 included ; yet it must be said that the great benefit of it, from 

 the photographic point of view, is not admitted by some good 

 authorities. Many of the appearances which he obtains and 

 which he claims to be improvements in- the images, may in 

 fact be due to other causes. 



1 Journal of the Royal Microscopical Society, February 1907. 



