1 60 Structure of Diatoms, &g. L^JoS. StTE' 
magnificently, yet for the ordinary class the inexpensive plan de- 
scribed in !N 0. YII. may be employed, consisting of the truncated 
lens, therein shown, cemented to an ordinary glass slide with 
Canada balsam. This must be used in conjunction with the para- 
bolic condenser, and the rays rendered parallel by the bull's-eye. 
Instead of water, the top of the slide may be covered with a film of 
glycerine, which will not evaporate ; of course this does not influ- 
ence the total reflexion which now takes place from the upper 
surface of the glycerine. If now filaments of plants, animalcules, 
or hairs of animals be laid on the glycerine, they instantly appear 
luminous. For example, a mouse-hair is acknowledged to be a 
somewhat difficult object to show satisfactorily by ordinary means, 
either opaque or transparently ; but viewed by this method its 
structure is unmistakable. There is no false glare, and the pig- 
ment cells are seen in their true position in the interior of the hair, 
which in fact forms the actual receptacle for the light. 
But perhaps some of the most remarkable results are obtained 
with the Diatoms. Though these are generally mounted on the 
glass cover, yet it frequently happens that some have become 
attached to the surface of the under-slide. It is these alone that 
are visible by this principle of illumination. The small trun- 
cated lens is made to adhere to the under-side by a film of oil of 
cassia or cloves, and the parabolic condenser used as before directed. 
Most perfect and apparently detached fragments of the Diatom 
scale are clearly brought out, and its structure proved satisfactorily 
and in a more reliable manner than by any other mode of illumi- 
nation that I have yet seen, as it is only those parts of the scale in 
jactual contact where total reflexion is destroyed that are visible, 
consisting perhaps of part of the mid-rib, and a row of dots here 
and there, or square or triangular patches, according to the species, 
and very frequently a httle isolated group of only three or four 
nodules together. 
It was my intention to have illustrated some of these appear- 
ances with camera lucida drawings, but I have now no time for such 
pursuits ; but as the truncated lenses are inexpensive, I trust that 
some one may be at the pains of employing them as a means of 
investigating these objects. 
