THE PRESIDENT'S ADDRESS. 37 



only in comparatively recent years that accurate measurements 

 of the so-called striae on the diatomaceee have been made.* 



The deductions which Mr. Wright has postulated in the next 

 Art., No. 22, is a most important addition to microscopical 

 literature. Speaking of the Fresnel interference bands, i.e., 

 microscopical images formed by a small cone or beam, centric 

 or excentric, he says " that these lines are in no sense 

 images, but mere interference bands or fringes with no 

 definite focus ; that whenever thus really produced they are 

 a constant source of uncertainty and error, and to be got rid of 

 as far as possible by the use of large aplanatic cones ; that 

 when we use such cones we lose such fringes altogether and 

 get a real focussed image, true to the object so far as the 

 aperture and correction of the lens permit of its definition ; and 

 that this image is a dioptric image." 



Further on, he says " that the narrow cone and the diffrac- 

 tion theory stand or fall together." This statement is per- 

 fectly correct, provided that it is the diffraction theory as 

 enunciated by Abbe and his exponents which is meant; and can 

 this be wondered at, seeing the theory at its inception was not 

 even a logically sound argument ? 



With regard to Abbe's statement, quoted in this article, viz., 

 " Strictly similar images cannot be expected, except with a 

 central illumination with a narrow incident pencil, because this 

 is the necessary condition for the possible admission of the 

 whole of the diffracted light," let me put before you a simple 

 experiment. Place a P. Angulatum under an objective of '65 

 N. A., and illuminate it by a narrow central incident pencil ; 

 you will see neither structure nor spectra. Enlarge the in- 

 cident cone until it fills three-quarters of the objective, and you 

 will now see both spectra and the angulatum pattern. This 

 proves that the wide cone is a better condition than the narrow 

 incident pencil for the admission of diffracted light. 



I am perfectly aware of the imperfections of this brief review 

 on Mr. Wright's important articles, but it has been my endeavour 

 to discuss them fairly ; neither have I consciously slurred over 

 or omitted any difficulties or unexplained points for the purpose 

 of making the case appear stronger than it really is. The sub- 



* "M. M. J.," Vol. xv. (187G), p. 223. 



