510 



Transactions of the Society. 



upon the object unless the rays diverged from the diaphragm 

 towards the " condensing " lens. But by constructmg the diagram 

 (as here given) from the object as the starting-point, we readily see 

 that a light-source placed in the position indicated (c/) would 



Fic. 2. 



/I 



V- 



ii 



(S^ 



C/ 



Fig. 1, from Dr. Wollaston'a data. 



Length of tube 6 inches. 



Focal distance of object from lens "8 „ 



Distance of lens from diaphragm 4-4 „ 



Aperture in diaphragm '3 „ 



Eadius of lens to give the focal distance wanted . . .. '4 „ 



Diameter of front surface of ditto "4 „ 



c/, Conjugate foci by construction showing diverging pencil from 

 light-source. 



Fig. 2, copied from p. 49, ' Encycl. Brit.,' Sir David Brewster's article on 

 Microscopes. 



Focal distance of object from lens 1-14 inch. 



Distance of lens from diaphragm 4-7 „ 



Aperture in diaphragm "2 „ 



Eadiusoflens '5 „ 



Diameter of front surface '6 „ 



Both figures show the effect of tlie diaphragm aperture in determining conju- 

 gate foci and divergence of pencil. Fig. 2 is, however, not drawn to scale 

 accurately in accordance with Dr. Wollaston's data, and therefore exaggei'ates 

 the supposed error of focus. 



Dotted lines show the focus for parallel rays. Continuous lines, conjugate 

 foci. 



N.B. By this arrangement the upper surface of the condensing lens becomes 

 virtually the light-source, and the diaphragm aperture just over the mirror acts 

 on the same principle as in the Brewster illuminator. Obviously tlie best 

 arrangement would be to leave the mirror free and remove the diaphragm, 

 placing a revolving diaphragm with apertures just over the lens at d'. 



give the necessary outlines of the diverging and converging cones, 

 the apices of which shall be conjugate foci. But Sir D. Brewster's 

 illuminator effects nothing more, and in passing judgment on the 

 position of the diaphragm in Dr. Wollaston's apparatus, he lays him- 

 self open to objections of a graver kind against his own arrangement. 



