^o^un^uMa™^.] Royal Microscopical Societij. 141 



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, beau- 

 tifully illustrated by sketches from life by the camera lucida, form 

 a classical paper in the ' Philosophical Transactions.' But a labo- 

 rious mquiry, chiefly conducted by means of the microscope, into 

 the limits of human vision, as determined by the nature of hght 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 sj)irits 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 pubhcation, he 

 learned that the Astronomer Koyal, Professor Airy, 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 " Eemarks on A. Boss's suggestion for three glasses 

 to admit a large pencil, which J. J. L. thought would not answer. 

 A. E. tried it, and found it a failure, before trying J. J. L.'s sug- 

 gestion 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 Boss's celebrated i-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. TuUey's an achromatic combination 

 made by him at Dr. Goring's suggestion, 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 

 xV>' 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 Microscopes,' read 

 before the Eoyal Society, Jan. 21st, 1830, announcing the dis- 

 covery of the existence of two aplanatic foci in a double achromatic 

 object-glass. This has formed a basis for subsequent important 

 improvements, the object of which has always been to obtain 

 sharpness and achromatism over the field in the picture from a 



