On an Improved Method , &c. By Sir W. Sendall. 707 
the field ; and this can generally be effected with a fair amount of 
approximation to accuracy. The calculation is as follows. 
The line A B, in the figure, being given equal to ten inches, and 
B E 
B E being known by observation, the tangent — -= of the angle 
A _t> 
E A B, and hence the angle itself, becomes known ; whence also the 
angle E A F, which is double of E A B, is known ; and the linear 
value of the arc C B D can then be taken at once from the table of 
circular values, to be found in every collection of mathematical tables. 
Fig. 79. 
Substituting this value for the measurement E F, we obtain a quan- 
tity which accurately expresses the dimensions of the magnified 
image, due to a uniform distance from the eye of ten inches. 
By applying a similar calculation in every case, all measurements 
taken with the camera upon a plane surface can approximately be 
reduced to their proper values ; it would, however, be much simpler 
and more satisfactory, where accurate results are of importance, to 
