236 THE MICROSCOPICAL NEWS. 
NACHET’S BLACK-GROUND ILLUMINATOR.* 
NDEPENDENTLY of the use indicated by the author it is 
possible to employ this piece of apparatus as a means of 
procuring oblique rays with objectives of all powers from the 
No. 2 upwards. 
When using artificial light, such as a paraffin lamp furnished 
with a shade, it is necessary to render the rays parallel by means 
of a bull’s eye condenser, and by raising or lowering the source of 
illumination until a uniformly illuminated field is obtained. The 
obliquity of the rays is produced in a special manner, and some- 
times the oblique rays proceed from all sides at once, lighting at 
one and the same time the upper face of the object which they 
skim over, and the lower face which they traverse. 
This illuminator, however, without being very difficult to. 
manage, requires, nevertheless, some care and management, espe- 
cially in the adjustment of the cone of rays, the disposition of the 
mirror, and the intensity of the light. The effects yielded by the 
employment of this method will be sure to astonish all those who 
have only employed ordinary light for the examination of objects, 
or have only used condensers of the ordinary pattern. Thus, by 
this means, I have been able to see :— 
1. With Nachet’s No. 2 objective, and No. 3 ocular, the draw 
tube out, the two systems of diagonal striz on Pleurosigma 
angulatum, 
2. With objective No. 3 and No. 3 ocular the transverse striz of 
Surirella gemma, which, without this method of illumination, 
required objective No. 5. 
3. With No. 5 objective and No. 2 ocular, the longitudinal 
strize on the same test. 
4. And in employing the oculars Nos. °3 or 4 with the same 
objective it was possible to resolve very clearly the hexagons of 
Pleurosigma, and to see the longitudinal sinuous lines of Surirella 
in a much better manner than with the No. 7 immersion objective 
used without this illumination. 
The results above described have been obtained so perfectly 
that persons completely ignorant of the disposition of the striz 
have faithfully described and figured them without having pre- 
viously seen any description or figure, and this enables me to state 
without the least doubt, or the least fear of error, that this 
illuminator, well managed, doubles in many instances the power 
of objectives. 
B. BuSSEREAU. 
* Note sur I’ eclairage a fond noir de Nachet. Translated from the Journal 
de Photo. et Micr., &c. 8e Annee No. 2. 
