18 
the origin of the Sargossa weed : some holding that it was produced in 
the Stream ; others maintaining that it was drifted from the low keys 
and islands of the Gulf. 
OcTroBER 27TH, 
MICROSCOPICAL MEETINGS.“ ILLUMINATION.” 
Mr. Henwan spoke of his experience of the facility with which 
sections of bone could be made and mounted by Dr. Ormerod’s process, 
as explained by Mr. Wonfor at the previous Meeting. 
Mr. Wownror remarked that, since the last Microscopical 
Meeting, he had made experiments with other hard substances, such 
as the stones of fruits and the shelis of nuts, and had found the 
process equally applicable. The great advantages over other methods — 
might be expressed in two words : simplicity and rapidity. 
Mr. Hewnaz then introduced the subject for the evening, ‘‘ Illu- 
mination,” by remarking that much more depended on illumination 
than on instruments or objectives. Good objectives, without proper 
illumination, failing to perform satisfactorily ; while, with it, inferior 
‘objectives could be made to do a great deal. In the illumination of 
objects by transmitted light it was very important that the rays of 
light should be rendered parallel before they arrive at the illuminating 
apparatus, whether mirror, condenser, or prism. By a proper atten- 
tion to this almost any structure could be fairly made out by the 
mirror alone, without expensive substage appliances. There should 
not be an excessive angle of illumination. It should not exceed 90°, 
or confusion instead of precision might be the result. In all 
substage appliances, the light should be exactly focussed on the i 
object, a point to which sufficient attention was not always 
paid. For thick substances direct rays were necessary ; but for 
thin and lined objects like diatoms oblique rays were essential. He 
then discussed the different modes employed for direct or oblique 
- illumination, such as the ordinary or flat mirror, the Nachet, and 
Amici prism, the achromatic condenser, and the Reade’s prism, among 
