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IV. On the concentric adjustment of a triple Object-glass. By 
William Hyde Wollaston, M. D. V. P. R. S. 
Read December 13, 1821. 
Hav ing in my possession a telescope with a triple object- 
glass of forty-five inches focus, made by Dollond in 1771, I 
have had a good opportunity of examining the central adjust- 
ment of the lenses, and have made trial of a method of cor- 
recting that adjustment, which appears not to have been used 
for that purpose. 
When I ventured to take to pieces an instrument that had 
stood the test of fifty years trial, with uniform approbation of 
its performance, those who knew the telescope, and who know 
the difficulty of centering, seemed to consider it an act of rash- 
ness which I was likely to regret ; but, by the test which I 
am about to describe, I felt confident that my object-glass 
was capable of improvement, and I rested my hopes of suc- 
cess on principles that seemed indisputable. 
When any bright object is viewed through a glass of this 
construction, without an eye-glass, there may be observed, at 
the same time with the refracted image, a series of fainter 
images, that are formed by two reflections from the different 
surfaces ; and, as the position of each of these images is de- 
pendent on the curvatures of that pair of surfaces by which it 
is formed, they appear at different distances from the object- 
glass. 
Since the number of surfaces is six, the number cf binary 
