580 



Transactions of the Society. 



unreal sharpness and tenuity of the hairs ; next place a stop at 

 the back of the condenser and view the same hairs on a dark ground, 

 when they will be found to present a swollen or thick appearance. 

 The true shape of the hairs lies of course between these two micro- 

 scopical pictures. 



The size of the antipoint is governed by the wave-length and 

 by the size of the utilised or working aperture of the objective, in 

 other words by A, and W. A.* 



Fig. 141. 



Fig. 142. 



The following table gives the amount to be added to the size 

 of the image on a bright field, or subtracted from the size of the 

 image on a dark field when accurate determinations of the magni- 

 tude of minute microscopical objects is required. 



The first column is for white light, the value of X being the 

 latest measurement by Mr. J. W. Gifford of the point of maximum 

 intensity in the spectrum. The second column is to be used when 

 a Gifford or similar screen is employed, X being taken for a 

 point between the lines E and F. The third column is for photo- 

 graphic work. To illustrate the method of applying this correction, 

 let us take the measurements of the Bacterium termo and its 

 fiagellum by Dr. Dallinger.t The mean of 200 measurements 

 gave -000004885 (201*700) in - for tne thickness of the fiagellum, 

 and -00004885 (20470) for tlie diameter of the tcrmo. Now, 



* For the method of determining the W.A., see J.K.M.S., 1901, p. 244. 

 t J.E.M.S., 1878, p. 175, pi. 8, fig. 1, pi. 9, figs. 10, 11, 12. 



