190 NATURAL HISTORY MISCELLANY. 
transparent object, the finer points are apt to be “drowned” out 
entirely; and it is only by modifying the amount of light by 
means of the diaphram, that we are enabled to make out the more 
delicate details. Hence it will be found that the use of the bull’s- 
eye condenser, for concentrating the light on the mirror, and con- 
sequently augmenting the amount of light passing through the 
object, is, in general, totally unnecessary. This arrangement of k 
the illuminating apparatus is totally different in its effects from 
that of the achromatic condenser, and cannot be substituted for it, 
as some persons seem to think. | | 
No man can have worked long with the microscope without be- 
ing led to a very careful consideration of the relative value of the — 
various sources of illumination at his command. Much thought, — 
and considerable experience, have led me to the following conclu- a 
sions on the subject :— “a 
The first requisite in the light that we use is whiteness. Hence, — 
daylight, the light of a white cloud, the artificial white cloud ilu- 
minated by daylight, the light from the old-fashioned argand lamp — 
burning sperm oil, the modern student lamp burning kerosene oil, 
and its various modifications, and the argand gas-burner, arè — 
good— their excéllence being about in the order here laid down. 
Common gas-light, candles, and kerosene lamps are inferior just 
about in the order we have named. White light is not nearly 80 
fatiguing to the eyes as the reddish glare from a half-smothered — 
combustion. Hence in all cases we must seek to have the most 
perfect combustion and highest possible temperature of flame mi 
our sources of artificial light. It is true that this gives rise to 
great heat, but this difficulty is easily obviated by the use of a : 
proper screen or shade, and none will be found better than the 
one previously described. Indeed, when working by artificial 
light, it will be found that the heat is one of the most efficient 
causes of injury to the eyes, and the screen that we have men: ’ 
tioned is, perhaps, quite as useful, from the fact that it cuts "m 
heat, as from its excluding unnecessary light. - 
The second requisite is steadiness. Nothing is more trying w 
the eyes than a flickering light. Sah EE 
Another requisite is that the instrument should be so steady 
that the object shall be retained in view and in focus without ? 
change. Any tremor is injurious to the eyes, and especially ie? a 
this the case when that tremor produces a continual change m +% 
