A NOTE ON MR. "IVIERLIN's PAPER. 115 



which, though known for many years past, has never received 

 any very great attention from microscopists ; I refer to the 

 enhancement of the resolution by the introduction of an analysing 

 Nicol prism into the path of the rays ; in this case, for con- 

 venience, between the eyepiece and the objective, although its 

 effect is apparently the same wherever it is placed. Without this 

 device, I have been unable to see the striae, even with the greatest 

 possible obliquity in the incident light, with a flame-edge in use ^ 

 I have seen them faintly, but certainly, with a gas-mantle and 

 strongly with a Nernst lamp ; but the effect of the introduction 

 of the analyser is, in the present case, most striking ; the resolu- 

 tion is perfectly plain with any illuminant in use, and there is not 

 the smallest difficulty (due to their faintness) in measuring them ; 

 the result, in the case of the specimen shown, being 115,000 to 

 the inch. 



The light diffracted by diatom striae is found to be partially 

 polarised, to an extent depending (according to Mr. Conrady) on 

 the nature of the structure, and on the angle between the incident 

 and diffracted beams — that is to say, on the fineness of the stria- 

 tion ; and it is by no means with all such objects that the effect 

 is noticeable. The same authority points out that the fact of 

 the diffracted beam being polarised proves that the object cannot 

 be considered to behave as if it were self-luminous, as is claimed 

 by some of the opponents of the Abbe theory of microscopic 

 vision. Probably the effect of the polarisation in enhancing the 

 visibility of the striae, in cases where resolution occurs, is due 

 partly to the reduction in the intensity of the direct beam, the 

 diffracted beam being little, if at all, affected ; and partly, no 

 doubt, to some relation between the planes of vibration of the 

 light-waves in the two beams. The prism is placed with its 

 longer dimension parallel to the length of the valve, and at right 

 angles to the striae ; and although, as said above, the effect is 

 by no means invariable, I think this device is at all times worthy 

 of trial in the case of difficult resolutions. 



In effecting difficult resolutions such as the case under con- 

 sideration, it seems necessary to have an illuminant of consider- 

 able intrinsic brilliancy ; i.e. very bright in proportion to its 

 size ; and the beam of light directed on the object should be as 

 narrow as possible, and at the extreme margin of the back lens 

 of the objective. A Nernst lamp gives an excellent result, and 



