534 REPORT—1896, 
case the correction is so small that for practical purposes it might be 
neglected, as on a No. 2 B.A. thread it only represents about 7,',jth of 
an inch, and proportionately less on the smaller sizes. 
However, to determine practically whether this was the case, I 
suggested filing away a part of the screw to be photographed to make it a 
comb. There could be no doubt that the photo of such a comb would 
give the true shape of the thread. Mr. Stroh kindly supplied such a 
screw. From the photos it would appear that there is no practical differ- 
ence in the shape, and that therefore the photograph of a complete screw 
gives a true record of the shape as well as the dirfiensions. 
The following gives the detail of the method employed in photographing 
the screws :— 
On a plate A, B, c, D, fig. 2, is a block, £, to which is secured a glass 
scale, Fr, which has been carefully etched with lines 4, inch apart. A piece 
of spring brass, HG, serves to hold the screw KK to be photographed, with 
its axis parallel to the plate a, B, c, D. The distance of the screw from 
the plate can be adjusted by means of the screw 1, so that when the scale 
FIG. 3. 
is sharply focussed on the screen the shadow of the screw may also be 
brought into focus. 
The frame 4, B, C, D, fig. 2, is adjusted in the same position as an 
ordinary microscopic slide in a magic lantern arranged for microscopic 
work ; only I found it desirable to employ a photographic lens of modern 
design instead of the usual objective. 
The arrangement is shown in the accompanying diagram, fig. 3, where 
A represents the limelight ; B the ordinary condenser ; c Alum trough 
to stop the heat rays; D Small condensing lens ; E Screw, in its 
frame as described in fig, 2; F Photographic lens ; c Milled head 
screw for focussing the lens F. 
APPENDIX II. 
Gauges for Verifying the Accuracy of Screws (for Workshop Use only). 
By A. Strou. 
In the gauge represented in fig. 4 the hole @ is for the external 
diameter. It should be of exactly the diameter given in the table of 
