528 Mr. Babbage on electrical and magnetic rotations. 
In some instances I used a plano-convex lens cut into 
halves, one half being fixed as much before the other as the 
distance of the needle from the graduations required ; this 
brought the image of the glass thread and the graduations 
into the same plane ; but as the lens I had cut was of too 
short focal length, this plan was not often adopted. 
I occasionally employed a narrow ring of glass, on one half 
of which the graduated card circle was pasted ; the back of the 
glass ring was blackened. By placing the eye above the needle 
in such a position that the direct image of the glass thread 
covered that which was reflected from the black glass, all 
parallax was avoided ; and I found that, without great care- 
lessness, repeated observations never differed more than a 
quarter of a degree. 
Another method which I made use of was, to draw a fine 
line along the needle ; and having made a small hole in the 
centre of the glass plate which covered the apparatus, I fixed 
an index of card, which could revolve about it as a centre ; 
this index extended to a graduated circle fixed on the same 
glass cover. When I wished to observe the position of the 
needle, I brought one edge of the index to coincide with the 
line on the needle. There were some advantages in this me- 
thod, but it had the inconvenience of obliging me to touch the 
apparatus at each observation. 
