550 OBITUARY NOTICE OF JOSEPH JACKSON LISTER 



together, I think I have met with nothing to equal it — the distance of the front 

 glass from the object being oii full.' 



Having now completely satisfied himself of the applicability of his principle, 

 he devoted much of his leisure for several years to various investigations by aid 

 of the instrument which he had so greatly improved. Some of the results 

 are well known to the public. Selections from his observations on zoophytes 

 and ascidians, beautifully illustrated by sketches from life by the camera lucida, 

 form a classical paper in the Philosophical Transactions. But a laborious inquiry, 

 chiefly conducted by means of the microscope, into the limits of human vision, as 

 determined by the nature of light and of the eye, has never been published. He 

 had at one time almost prepared an account of it for the press, when the illness 

 of his eldest son, which ended fatally, threw such a cloud over his spirits that for 

 several years he had not the heart to complete the work. And when at last 

 he did resume it, and was on the eve of publication, he learned that the Astro- 

 nomer Royal, Professor Airj^ had reached the same conclusions, though by 

 a different road, and so abandoned the idea, a circumstance in my opinion 

 deeply to be regretted. 



But to return from this digression. The next note in order of date regarding 

 the construction of the microscope is one made in 1837, headed ' Remarks 

 on A. Ross's suggestion for three glasses to admit a large pencil, which J. J. L. 

 thought would not answer. A. R. tried it, and found it a failure, before trying 

 J. J. L.'s suggestion below.' Then follows a drawing of a proposed combination 

 of three glasses ' for the same object ', giving the dimensions of the lenses and 

 the curves of the various surfaces, with a statement of the effect proposed to 

 be produced by each glass upon spherical aberration and coma. This resulted 

 in Ross's celebrated f- inch object-glass, the construction of which was afterwards 

 adopted by the other principal London makers. 



A statement in his handwriting found among his papers gives, in a few 

 words, his relations to the British microscope : — 



* I had been from early life fond of the compound microscope, but had 

 not thought of improving its object-glass till about the year 1824, when I saw 

 at W. Tulley's an achromatic combination made by him at Dr. Goring's sug- 

 gestion, of two convex lenses of plate glass, with a concave of flint glass between 

 them, on the plan of the telescopic objective. They were very thick and clumsy. 

 I showed him this by a tracing with a camera lucida, which I had attached to 

 my microscope, and the suggestions resulted in " Tulley's y\ ", which became 

 the microscopic object-glass of the time. But the subject continued to engage 

 my thoughts, and resulted in the paper On the Improvement of Compound Micro- 

 scopes, read before the Royal Society, on the 21st of January, 1830, announcing 



