624 SUMMARY OF CURRENT RESEARCHES RELATING TO 



tracing back the amount of light which can enter the eye-piece, it is 

 again found that the marginal zone of the objective is cut off, and that 

 too low a result will be obtained in this case also. 



The remedy in all such cases is to do away with the auxiliary 

 Microscope altogether, and to observe the back of the objective to be 

 measured by looking directly at it down the Microscope tube, for as this 

 danger only exists in the case of unusually large clear apertures, it is 

 quite easy and accurate to observe without a magnifying instrument. 



Numerical Aperture and Resolution. — A neat contrivance was also 

 shown to demonstrate the fact that resolving power increases with 

 aperture. The instrument simply consisted of four pieces of wire 

 netting, graduated in fineness and mounted in a suitable frame, and 

 of a plate with four perforations of different diameters, placed at a 

 suitable distance from the frame, so that the latter might be observed 

 by looking through any of the perforations. Tlie distance between 

 screen and perforations being adjusted so that when looking through the 

 largest perforation all four meshes could be distinctly seen, it was found 

 that on looking through the next smaller perforation only three of the 

 meshes were resolved, the finest appearing without detail ; the still 

 smaller perforation would resolve only the two coarser meshes, and the 

 smallest perforation would only show the coarsest of all. A complete 

 analogon was thus provided of the effect of aperture in either telescope 

 or Microscope. 



Ordinary versus Compensating Eye-pieces. — In order to demonstrate 

 the difference between ordinary and compensating eye-pieces, two 

 Microscopes were set up side by side, one fitted with a strictly achro- 

 matic objective, calling for an ordinary eye-piece, the other fitted with a 

 Holos objective, requiring a considerable compensating effect in the eye- 

 piece. Two Holos eye-pieces were accurately adjusted, one to suit the 

 achromatic, the other to suit the Holos objective, and visitors were 

 invited to notice the effect of exchanging these eye-pieces, in order to 

 dispel the rather prevalent idea that there was some special virtue in com- 

 pensating eye-pieces to which a great part of the excellence of modern 

 objectives was due ; the exchange producing either good images in both 

 Microscopes or else bad images in both Microscopes, the latter being 

 characterised by the appearance of broad coloured fringes on the edges 

 of the silver lines of the Abbe test-plates which were employed as objects ; 

 and it was pointed out that the only special effect produced by the com- 

 pensating eye-piece was that it has a higher magnifying power for a red 

 object than for a blue object, and as the modern high-power objectives 

 with unachromatic thick front lens have a similar difference of magnify- 

 ing power in the opposite direction, the compensating eye-piece, if 

 properly adjusted, will produce an image free from colour fringes on 

 such objectives. 



Tests for Objectives. — Another four Microscopes were shown demon- 

 strating the use of the Abbe test-plate for examining objectives and 

 determining the nature of their defects. 



The first of these instruments showed how spherical aberration can 

 be readily detected by shifting a comparatively narrow cone of light 

 gradually from the central to the marginal zone of the objective under 

 test, either by using the usual turn-out ring of the substage, or, more 



