10 REPORT—1840. 
part of the microscopic object to be observed, so that the illuminating 
rays may radiate, as it were, from the object, as if i¢ were self-luminous. 
Now this can only be well obtained by illuminating with a singlelens, ora 
system of lenses, without spherical or chromatic aberration, whose focal 
length, either real or equivalent, is less than the focal length of the 
object-glass of the microscope. The smaller the focal length of the 
illuminating lens or system of lenses, the more completely do we secure 
the condition that the illuminating rays shall not come to a focus either 
before they reach the object, or after they have passed it. When Dr. 
Wollaston recommended for an illuminating lens, one of three-fourths 
of an inch in focal length, in which the microscopic object was placed 
in a vortex of foci, where the rays crossed in a thousand points both 
before and after they fell upon the object, he could have had no 
idea of the new method of illumination. In the construction of a per- 
fect microscope, Sir David Brewster recommended that the illumina- 
ting and magnifying apparatus should have separate and similar move- 
ments along the same rod or bar, and that the stage for the objects 
should be unconnected with either, and should have also a motion 
independent of both. 
On an Improvement in the Polarizing Microscope. 
By Sir Davip Brewster. 
This improvement consists in placing the analysing prism or simple 
rhomb immediately behind the object-glass, that is, on the side of the 
object-glass next the eye. The great inconvenience of placing it be- 
tween the eye-glass and the eye, had induced several skilful observers 
to reject the prism altogether as an analyser, or to substitute for it a 
plate of tourmaline, which is quite unfit for any observations in which 
colour is to be considered. The analysing prism may remain constantly 
on the microscope behind the object-glass, without in the least injuring 
the performance of the microscope, and it should have a motion of | 
rotation independent of the body of the microscope. 
Extract of a Letter from Col. Rei to Sir D. Brewster. 
Bermuda, August 17th, 1839. 
Dear Sir,—I think the accompanying letter, which describes the singular ap- 
pearance of the sun at Bermuda, which made white objects appear blue, cannot | 
fail to interest you; and if you are able to explain the cause of this, I should | 
be very glad if you would favour me with such explanation. The fact is one | 
familiar to every one here; but I requested Dr. Harvey to put it in writing, / 
expressing what he saw himself, that I might send the account to you. The 
present collector of the customs at Bermuda was at sea on the 11th of August, the | 
day the same hurricane was passing over St. Vincent; and to him and to the | 
other persons on board, objects appeared, they thought, of a light green or 
bluish green colour, and the sun had this same appearance. Their vessel was | 
then fifteen miles east of Bermuda. ‘The hurricane reached Barbadoes a little | 
before midnight on the 10th of August, 1831. / 
