300 MEMORANDA. 
their favorite objects, and who can easily contrive arrange- 
ments for holding glasses of various shades and colours. 
In many instances, with artificial illumination, the best 
effect would be obtained by throwing the most intense light 
possible on the object; but in this case the light fatigues the 
eye and destroys its sensitiveness. It is then usual either to 
lower the achromatic condenser, or to employ a lamp with a 
blue shade; but by placing a light moderator of coloured 
glasses over the eye-piece, the object may be examined with 
the strongest light obtainable—F. H. Wrenunam. 
On Coloured Illumination—Since Mr. Wenham called my 
attention, a few days past, to the employment of strong 
sunlight passed through an achromatic condenser, behind 
the object, and coloured glasses over the eye-piece, im 
the examination of various objects, the markings of which 
under the ordinary method of illumination are somewhat in- 
distinct, I have made a few trials with such colours as were 
to hand. They were not of a sufficiently extended character 
to enable me to say in what particular class of objects the 
method may be found most useful, therefore these remarks 
may prove of very little value. 
The external contour of many pale objects appeared more 
defined; in insect structures the internal parts were more 
distinct, with bolder shades giving contrast relief to the 
other parts. It appeared requisite to seek the coloured glass 
most useful to appropriate, what I would call, the false light, 
otherwise in many objects it did not offer anything striking. 
The coloured glasses tried were light orange, medium or 
rather blue, intense green, intense neutral tint, and carbuncle 
red. They werevariously arranged. The first two, used together, 
gave a very pleasant tint to the field, but I do not think they 
were of sufficient intensity to permit of long employment 
without fatigue to the eye. Many of the diatoms gave their 
interstructural lines very distinctly. Intense green alone had 
a fine effect in Aulacodiscus, Arachnoidiscus, Heliopelta, 
cotton-fibre of Zostera, &c. Deep neutral tint appeared 
useful in the examination of the paler kinds of Acari, the 
orange colour gave to the deeper coloured Acari a depth in 
their structural parts. ‘The carbuncle red scarcely permitted 
any structures to be seen. The same glasses placed behind 
the condenser, also behind the object, were not equal im 
effect to when placed over the eye-piece. The neutral tint, 
however, seemed to throw up some structures with more defi- 
