346 APPENDIX. 



greater than the loss by absorption in its transit through the cupreous 

 solution. The details of this judicious and well-directed experiment 

 testify to the care bestowed on these matters at the Government 

 Laboratory. Dr. Woodward is about to institute a series of compara- 

 tive experiments between the lenses of various makers, and between 

 the lenses employed dry and a r immersion. In a photomicrograph 

 sent to Dr. Maddox by M. Nachet fils, as taken with his No. 7 objec- 

 tive by M. Lakenbauer, the areas of Pleurosigma angulatum are 

 shown as nearly circular with a central dot, considered by these 

 observers to be the foci of the little hemispheres. 



on p. 232 it should have been stated that in 1851 Dr. Hugh 

 Diamond, in conjunction with Mr. Archer, succeeded in taking 

 photographs of microscopic objects by the collodion process. 



