PROCEEDINGS OF SOCIETIES. 
57 
the beam of light, details could be seen with the poorer lens as well 
as with the better one, with the same obliquity of illumination and 
with a less obliquity with the best lens. Also, if yellow glass were 
used, a greater obliquity was necessary with both lenses. Also details 
were better seen with sunlight than with kerosene light, better with 
kerosene than gas, better with gas than with a candle ; all with the 
same obliquity of illumination. The blue colour most favourable for 
illumination he found to be that answering to the point in the spectrum 
where the maximum of chemical force or actinism resides. From 
this he framed a theory of vision, that it was due to the chemical 
action, and further experiment and experience tended to confirm this 
conclusion. The best way, he found, to get such monochromatic 
illumination was by means of a prism properly mounted. Evidently 
sunlight was the best to use in microscopic work, kerosene next. He 
begged to remind the members that colour blindness, or the ina- 
bility to distinguish colours, was by no means uncommon, and that 
the eyes were not always to be trusted. He called their attention to 
the fact that a kind of light could be used to illuminate objects that 
would change their apparent character very materially, or even make 
them disappear entirely, and, in illustration, illuminated a coloured 
chart of the spectrum by means of gaslight, kerosene light, sodium 
light, and magnesium light. In the sodium light all the colours but 
yellow disappeared ; whilst in the magnesium beam certain colours 
appeared that could not be seen in gaslight. He also showed how 
colours could be tested by means of the spectroscope, and said that 
this was one of, if not the best method of testing the correction for 
colours of microscope objectives. In this way he showed that this 
now mucli-talked-of “ mazarine blue ” glass was not blue at all, but 
more purple, as it transmitted a large amount of red light along with 
blue aud other colours. 
The regular meeting of the San Francisco Microscopical Society 
was held on Thursday evening, March 1, with President Ashburner 
in the chair. 
Professor Ashburner donated a slide mounted by him with ex- 
foliations from glass vases found in old Greek tombs, which showed 
some very curious features. 
Mr. Kinne donated a slide mounted by him with the scale of 
salmon as a polarizing object, which was found to be of a character 
that would cause it to rank high among those beautiful and instruc- 
tive objects. Mr. Kinne also presented two varieties of siliceous 
sponges from Oakland and Pescadero Beach, and a series of slides 
mounted by him with sections and fragments of the same, as well as 
the spicules cleaned from any extraneous matter to show their peculiar 
forms. Mr. Kinne stated that while they could not be called glass 
sponges in the strict sense of the term, from the fact that a certain 
amount of the framework or skeleton was composed of horny material, 
still the latter seemed to have only been used as a cement to hold the 
points of the acicular spicules together in one case, and as a very 
slight integumentary deposit about masses of spicules arranged in a 
filamentous form which anastomosed and spread in every direction, in 
