208 SUMMARY OF CUKKENT RESEARCHES RELATING TO 



for up-and-down movements. The toothed bar Z carries at its lower end 

 a holder H with a universal clamp K, for receiving and setting in suit- 

 able positions such pieces of apparatus as very fine lancet points, couch- 

 ing needles, stimulation-electrodes, and so on. The examination of some 

 objects would be suitably carried out in a small circular trough to whose 

 rim the object has been secured by a special clamp Scb. Very small 

 objects could be secured by white of egg, or by a gelatin layer on an 

 object-slide. In all cases the application points of the instruments must 

 be first focused and brought into the centre of the field. The tube is 

 then racked down and the object also brought into the centre of the 

 field. If, for example, it is desired to sever an object, the point of the 

 lancet is brought into the necessary lateral position and the operator 

 rotates the whole apparatus, the result being a microscopical section of 

 required length. By means of the milled head T the depth of the sec- 

 tion can be regulated. The simultaneous use of an eye-micrometer is 

 of advantasfe. 



(5) Microscopical Optics and Manipulation. 



Relation of Aperture to Power in the Microscope Objective.* 

 E. A. Hutton, in discussing this subject, keeps steadily in view the con- 

 siderations which should guide a microscopist in the selection of a lens, 

 and whether a purchaser would, for example, make a " better bargain " 

 in acquiring a ^-in. objective of N.A. 0*88, or of N.A. 0'74. The 

 complete answer partly depends on eye-piece magnifying power, and 

 partly on the average power of resolution for ordinary eyesight. These 

 he respectively takes as 10, which all modern high-power objectives w^ill 

 stand, and 125 lines to the inch. Again, an objective of 0'13 N.A. 

 will, according to the R.M.S. tables, resolve 12,500 lines to the inch. 

 To accomplish this the objective must have an initial power of 10, 

 which, multiplied by 10 (the eye-piece power), and then by 125 (the 

 limit of resolution of ordinary eyesight), gives the above result. In this 

 way the author constructs the following table, which gives the minimum 

 N.A. required (tube-length 160 mm.). 



It is quite true that owing to optical difficulties the first five in the 

 table cannot be constructed, but those objectives will l)e preferable 

 which approximate the most closely to these figure=!. Excess of aperture 

 is useless, and moreover it involves sacrifice of depth of focus. Applica- 

 tion of these figures to the disk of confusion gives, for example, by the 



* Knowledge, xxxvi, (1913) pp. 63-65. 



