THE PRESIDENT'S ADDRESS. 35 



No. 3. — Dark ground obtained by a condenser and a stop ; 

 this is only available for the lower powers ; the apochromatic 

 | or | of -65 N.A. may be said to exhaust this method of 

 illumination. This case, in my opinion, comes wholly under 

 the new theory, because all the aperture is uniformly utilized. 

 To all intents and purposes an object such as a diatom may be 

 said to be self-luminous ; under these conditions the action of a 

 microscope most closely resembles that of a celestial telescope. 

 The practical resolving limit is only a trifle below that of No. 2 ; 

 theory, however, demands that it should be higher. Taking the 

 same wave-length the resolution for No. 3 should, according to 

 the new theory, be 79,044 times the N.A. of the objective, 

 against 72,307 times the N.A. for case No. 2. Practice, how- 

 ever, as we have seen, reverses the order, and gives No. 2 a 

 slightly higher resolving power. The condenser must, of 

 course, have far more aperture than the objective. 



No. 4 comes entirely under the new theory, but resolution 

 falls off considerably; there is also an indistinctness in the coarse 

 structure. Theoretically the limit is the same as that of No. 3, 

 viz., 79,044 times the N.A. of the objective. This mode of 

 illumination is not practical. A condenser of larger aperture 

 than that for No. 2 is required, because it must, of course, equal 

 that of the objective. 



It is interesting to notice that with No. 2, the 3/4 cone illumi- 

 nation, if the object is placed at the edge of the image of the side 

 of the flame, especially if the edge is somewhat undefined by the 

 condenser being brought a trifle within or without its focus, the 

 resolving power is increased. This well-known illuminating 

 dodge becomes an important confirmation of the new theory, 

 for Mr. Wright, with reference to the action of the illumination 

 from a wide-angled cone, says, in Art. 23, that a " plenum " of 

 rays "in the same phase" diverge from each point in the 

 structure, so that the points become centres of wave propaga- 

 tion, but along the edge of the cone diffraction phenomena arise. 

 Does not, therefore, the above experiment fully confirm the 

 statement in Art. 23, which should itself be read, as it is far 

 better expressed than in my condensation. 



In this connection it will be found that a curious effect is 

 produced when examining fine-lined structures with the naked 

 eye, if an obstacle is held somewhat nearer the eye and the lined 



