1901.] NATURAL SCIEN'CES OF PHILADELPHIA. 323 



Aulacodiscu-s Oregamus is one of the few diatoms that show bright 

 colors with central transmitted light. The two valves of this 

 species included on slide under observation, when examined with a 

 three-fourths-inch objective of .25 IST. A., were bronze-yellow when 

 dry, yellowish gray in alcohol, bluish gray in medium of 1.41 

 R.I., iridescent blue in medium of 1.44 R.I., deep greenish blue in 

 cedar oil, dark green and pink in oil of cassia. 



The question of colors shown by diatoms in direct light has 

 recently been treated in the Journal of the Queckett Club, with special 

 reference to Actinocyclus Ealfsil, by E. jM. Nelson, who has shown 

 that the color cannot be due to diftraction. The two valves of 

 A. Ralfsii which were included in the previously described slide 

 showed only pale brown and grayish tints in media of R.I. below 

 1.50, and extinguished with the other forms in one of R.I. about 

 1.48. In cedar oil one valve showed a blue color and in oil of 

 cassia both became brilliant with green, blue, purple and yellow. 

 Under wide aperture objectives the color is not visible when diatom 

 is sharply in focus, but appears as soon as thrown slightly out of 

 focus. This color appears to be due to dispersion, and its nature 

 and cause might possibly be further elucidated by studying the 

 effect produced by different media such as were employed in this 

 case. 



May 28. 

 ]\[r. Arthur Erwin Brown, Vice-President, in the Chair. 



Eighteen persons present. 



Papers under the following titles were presented for publication: 

 " Contributions to the Life History of Plants, No. XV," by 



Thomas Meehan. 



" Observations on the Placenta and Young of Dasypus sex- 



ciuctus, " by Henry C. Chapman, M.D. 



The death of Dr. D. B. McCartee, a corresj)ondeut, July 1, 

 1900, was announced. 



]Mr. Adolph Fredholm was elected a member. 



The following were ordered to be printed: 



