ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 333 



MICROSCOPY. . 



A. Instruments, Accessories, etc.* 

 (3) Illuminating' and other Apparatus. 



An Addition to the Microscope.f — Under the above title M. A. 

 Ainslie describes an auxiliary, which, placed behind the objective, 

 facilitates correction for cover-glass thickness and for tube-length. If 

 a correction collar be applied to an objective it no doubt obviates to a 

 large extent the change of tube-length without interfering much witli 

 the magnifying power, and is useful in other ways, as, for example, in 

 focusing through the various planes of a thick object ; but, as it un- 

 fortunately appears to be going out of use except in the dry apochro- 

 mats and in water-immersions, the author leaves it out of consideration. 

 Under the name of a " transformer " the late Van Heurck applied 

 behind the objective a lens of small power, either convex or concave, 

 according to the effect desired, and stated that in this way he was able to 

 use even a 2-mm. apochromat, corrected for the short tube, on the large 

 tube, without any appreciable loss of definition. The explanation is 

 that if a convex or concave lens of low power be introduced immediately 

 behind the objective, it has the effect of altering the degree of con- 

 vergence of the rays of light projected by the back lens of the objec- 

 tive, and thus of altering the position in which the image is formed. 

 Conversely, if the objective requires, to give good definition, that the 

 image should be formed in a plane either within or beyond the avail- 

 able limits of the tube, it is perfectly possible in the great majority of 

 cases to find a lens of such a power that its introduction above the 

 objective will bring the image within the limits of the tube. It is an 

 advantage to have the additional lens as near as possible to the back 

 lens of the objective ; but, if there is any difficulty in fitting it there, 

 it does very well to place it on the nose-piece. The author finds that 

 with objectives of not too high power there is scarcely any limit to the 

 amount of correction which can be thus produced. 



One result, which the author believes to be novel, is the possibility 

 by this means of converting an oil-immersion objective into a water- 

 immersion. If the auxiliary lens be suitably chosen, it is found that 

 only the extreme margin of the image is affected, and that with an 

 illuminating cone of about • 75 or 0*8 N.A. the central definition is 

 quite satisfactory. Here, again, the advantages of the method are more 



* This subdivision contains (1) Stands ; (2) Eye-pieces and Objectives ; (3) 

 Illuminating and other Ai)paratus ; (4) Photomicrography ; (5) Microscopical 

 Optics and Manipulation ; (6) INIiscellaneous. 



t Journ. Quekett Micr. Club, xii. (1915) pp. 5G1-7G. 



June 21st, 19PJ 2 a 



