Hall, 07i Paraffin Oils. 
87 
Behind tlie object-glass, at a distance of three or four 
inches, an adjustable slit («) is placed ; the object-glass is fo- 
cussed upon the object on the stage so that its magnified image 
falls precisely upon the slit. The opening of the slit, which 
may be from 2-5-0 th to 4-^ o^th of an inch, allows the light of a 
small part only of this image to pass on to the prisms. If 
desired, this part of the object may be further reduced by 
shortening the length of the slit. It is obvious that, by the 
usual stage adjustments, any portion of the object can be 
made to fall within the jaws of the slit and to form a sepa- 
rate spectrum. (See diagram.) 
Behind the slit, at its own focal distance, is placed an 
achromatic lens [b). The pencils emerge parallel, and then pass 
through one or more prisms (c). The pencils are then received 
by a small achromatic telescope id), with which the spectrum 
is viewed. 
The eye-piece of this telescope is adjusted so that the lines 
of Fraunhofer in solar light, or the sodium line in an artificial 
light from a source containing sodium, are well defined. 
The object-glass of the microscope is then to be moved 
towards or from the object on the stage until the longitu- 
dinal lines and bars of difi'erent intensity, due to the darker 
and lighter parts of the object, are sharply defined in the 
little telescope at the same time as the lines of Fraunhofer 
or the double line of sodium. 
Paraffin Oils : their relative Value to the Microscopist. 
By W. H. Hall. 
(Read May 10th, 1865.) 
{Abstract.) 
The author commenced by stating, that in order to test 
the relative value of certain condensers, he was desirous of 
trying their powers by a good and similar light, and was thus 
induced to compare the purity and relative value to the mi« 
croscopical observer of the three parafiin oils, known as 
" Young's Parafiin,^^ " Price's Belmontine,^^ and a Diamond 
Crystal Oil from America," as being always easily obtainable 
from any respectable dealer in these oils. The table an- 
nexed gives the result of two series of experiments, made with 
three lamps of similar construction, with flat wicks cut from 
the same piece of cotton, and with an equal quantity of oil in 
each lamp. The first experiment was made by placing the 
