58 Capt. Kater’s experiments for determining the 
knife edge, or from a line terminating a surface, are liable to 
much uncertainty from what has been called irradiation , or 
indistinctness of the image. But this is by no means the fact ; 
for if the reflection of light from the knife edge be prevented, 
and it be viewed on a white ground, it may be made to bisect 
the cross threads of the microscope, with nearly the same 
precision as could be attained by the use of a line. There 
is, however, a correction necessary to be applied in this case, 
and I shall proceed to describe the method employed for as- 
certaining its amount. 
A slip of writing-paper was pasted on the mahogany case, 
under each knife edge, extending beyond it about the tenth 
of an inch, and adjoining, was a piece of black paper to pre- 
vent the reflection of light on the knife edge from the sur- 
rounding objects. The knife edge now appeared through 
the microscope, as a well defined dark object on a white 
ground. 
Marks were made on the paper close to the knife edges 
at equal distances on each side of the bar. These were in- 
tended to indicate those parts of the knife edges equally 
distant from the middle, from which the measurements were 
to be taken. 
The knife edges being adjusted parallel to each other, in 
the manner before described, the microscopes were brought 
successively over such marks on the paper, as were at the 
same distance from the bar, and the mean of each pair of 
observations being referred to the scale, gave a distance of 
the knife edges free from any error which would be occasioned 
by a want of parallelism. 
The knife edges bisecting the cross threads of the micro- 
