1902] MICROSCOPICAL JOURNAL. 51 
side screw of 50 threads to the inch. Another piece of 
telescope draw-tube was selected about 6 in. long, which 
was made to slide easily in this jacket by grinding the two 
together with rotten-stone and oil. A ring about a quar- 
ter of an inch broad was soldered outside within an inch 
from the other end of this last-mentioned tube, and this 
ring was also cut with a screw of 50 threads to the inch, 
so that when this tube was placed inside the jacket and 
turned round, it could be made to approach or recede from 
the sensitive plate in the dark slide by a very minute 
quantity. To enable this to be done easily, this tube had 
a ring soldered on to its extreme outer end, fully a fin. 
larger in diameter than the tube. This ring was knurled 
on the edge, which enabled the fingers to grip it without 
slipping, while the inner end of the jacket, being sprung, 
enabled the tube to be rotated by means of the fine screw 
motion without the least apparent movement of the image 
on the screen while focussing. The inner end of the in- 
side tube had the universal microscope screw to take the 
object-glasses to be used. Both of the two latter men- 
tioned screws should be carefully and evenly cut, no drunk- 
en threads being permissible. Of course, the tube was 
blackened inside, and a stop placed to prevent reflection 
of light from the sides. This accessory is really a very 
simple piece of apparatus, and easily made. The dark 
slide used was a singleone. A carrier was made to fit it, 
having an aperture in the centre to take plates the size of 
_ a quarter-plate cut into twelve pieces. One of these plates 
was taken and obscured by grinding one side all over 
with fine emery. This was used for rough focussing, 
while another was taken and obscured by grinding the 
outside portions of one of the faces, leaving about a $ in. 
square in the centre untouched. This was coated witha 
very thin film of virgin wax, and was used for fine focus- 
sing. This wax film makes probably the best screen for 
fine focussing. The glass is rubbed over three or four 
a 
