THE LIGHT. 31 
For convenience in working, it is desirable that 
the microscope should be as nearly horizontal as 
possible. To secure the best illumination, it is 
essential that it face the clear blue sky. Theo- 
retically, those two conflicting requisites could be 
reconciled by the use of a plane mirror inclined 
at an angle of 45°, by which light from the sky 
overhead would be reflected in a horizontal direc- 
tion; but in practice I have not found this to 
work so well, especially when no condensing lens 
is employed, for the reasons that there is a loss of 
light in changing its direction by a reflecting sur- 
face, and that imperfections in the mirror or dust 
upon its surface give rise to an unequally illumi- 
nated field. An apparatus mounted upon hinges 
in such a manner that it could be placed horizon- 
tally or more or less obliquely as required, would 
probably fulfil the conditions the most satisfac- 
torily, as the adjustments could be made while 
it was in a horizontal direction, and the anterior 
extremity elevated, or the posterior extremity 
depressed, for the purpose of making the ex- 
posure. In this case, a final focusing would be 
necessary, after changing the position. 
The writer has not found it necessary to resort 
to either of these expedients, but has been in the 
habit of working with his instrument permanently 
inclined at an angle of eight or ten degrees. 
The instrument may be directed towards any 
point of the compass; but if it points south, the 
direct rays of the sun should be excluded by a 
short tube, blackened on the inside, arranged with 
