148 MICEOSCOPIC VISION. 



vol. ii. p. 291 (1869) there is a figure on a fly-leaf illustrating 

 some remarks on the Frauenhofer theory by Dr. Barnard, 

 the President of Columbia College, to v/hom Col. Wood- 

 ward had referred the question. This figure supplies one 

 of the points missing in Frauenhofer's argument. That 

 argument was, as we have seen, defective in these two 

 respects : first, no account was taken of oblique light ; 

 secondly, there was no provision made for media other than 

 air. Now, this figure of Dr. Barnard's supplies the first 

 of these omissions by giving the paths of the diffracted 

 beams arising from a pencil of parallel rays falling obliquely 

 on a grating. Dr. Barnard disagrees with Fraueuhofer's 

 conclusions, and says, " With an objective that takes in 

 a cone of an angle of from 140° to 175°, it is nonsense to 

 talk of this question as one settled by theory." He then 

 goes on to say that if Frauenhofer's theory is correct, not 

 even the 9th band of Robert's 19th band test-plate (56,300 

 lines per inch) could be resolved in monochromatic yellow 

 light. Col. Woodward states that at the time he had re- 

 ceived Dr. Barnard's letter he had resolved up to the 18th 

 band and subsequently the 19th with a P. and L. water im- 

 mersion y^-g. So we see that although these two eminent 

 men had a correct picture before them, they were unable 

 to perceive its significance. 



In fine. Col. Woodward's successful photographs had so 

 completely shattered the Frauenhofer theory that it was 

 not worth further consideration. You have now before 

 you the opinions of microscopists both as to aperture and 

 diffraction, at the time when the diffraction theory of 

 microscopic vision as demonstrated by Abbe and Helm- 

 holtz was given to this country by Dr. Fripp. It will 

 be unnecessary for us to make any prolonged examination 

 of the Abbe theory, for all are familiar with it, as it has 



