BIOLOGICAL AND MICROSCOPICAL DEPARTMENT 



OF THE 



ACADEMY OF :N'ATURAL SCIEl!^CES. 



January 16. 



conversational meeting. 



Prof. J. H. McQuiLLEN in the chair. 



Twelve members present. 



The attention of the Section was asked by Prof. James Tyson, 

 to a simple diagram which he had been in the habit of using in 

 his lectures, for several years past, to impress upon students the 

 circumstances under which the reversal of lisihts and shadows takes 

 place in red blood-corpuscles while under microscopic examina- 

 tion. The familiar " cracker shape" of the corpuscle being ac- 

 knowledged, and supposing the entire corpuscle in focus when 

 there is the least shadow, it is plain that the central portion is a 

 double concave lens, while the periphery will act as a double con- 

 vex lens. The centre of the corpuscle will therefore cause the 

 parallel rays r r r to disperse and pass beyond the corpuscle, 



