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On a New Method of Substage Illumination. 

 By Dr. J. Matthews. 



(Bead 27th May, 1870.J 



I must confess to a feeling of considerable difficulty in introducing 

 the subject of my paper of this evening to your notice. I had 

 been at work many months upon it, when it suddenly came to my 

 knowledge, about three weeks since, that I had fellow-workers in 

 the field, and that we had all arrived at results very nearly alike, 

 by similar means. Nay, more — that those means had been 

 hinted at or foreshadowed in the pages of Carpenter, Hogg, and 

 others, and even employed by Tollit and Davis. But my experi- 

 ments were then complete, so that I think I may fairly claim to be 

 at least one of the first who has employed these new agencies, and 

 applied them in a formal and convenient way to the microscope, 

 so as to facilitate their use and give precision to their results. 

 But previously to laying before you these results it may not be out 

 of place to notice briefly the present modes of substage illumina- 

 tion. 



First is the plane or concave mirror — reflecting daylight or lamp- 

 light, and in so doing absorbing about half the incident rays, to 

 the great detriment of its use with the higher powers, besides 

 giving images from both front and back surface so that the direct 

 light of the lamp was often substituted — an excellent plan, easily 

 applicable to almost all cases. 



Next is the prism, either rectangular or equilateral — bounded 

 by plane surfaces — first employed in the Newtonian telescope. 

 This was a great advance, as by it nearly all the rays are reflected, 

 and there was only one image given. Amici and Abraham curved 

 the two surfaces opposite to the reflecting face of the prism into a 

 lenticular shape, by which it became a condenser as well as a re- 

 flector ; a most valuable improvement, especially in that of Abraham 

 who made his achromatic. The " Diatom prism," of Mr. Reade, 

 is of the former kind. All these (except that of Mr. Abraham) 

 possess more or less of these faults ; that they do not reflect light 



