64 ReadEj on a New Hemispherical Condenser. 



diaphragms to stop out the light in the right place of a 

 Gillet's, or perhaps still better, from its greater angle of 

 aperture, a PowelPs condenser, we should approach perfection 

 in resolving difficult markings under the deepest powers. 



My old black-ground illumination, which led to the forma^ 

 tion of valuable condensers by Messrs. Shadbolt and Ross, 

 may be produced Avith very good effect by the hemisphere and 

 a single aperture ; and I feel sure that the members of our 

 society will be much pleased with the brilliant definition and 

 detail of a scale of Poclura under this illumination and the half- 

 inch object-glass. I have in my possession the same scale 

 which my old and valued friend, Andrew Ross, saw with his 

 first achromatic ^th, in his little workshop at St. John's, 

 Clerkenwell, and I shall never forget the expression of his 

 astonishment. But the present half-inch is superior in all 

 respects to that -^th. 



It is now generally known that I offer the hemispherical 

 condenser as the special adjunct of the new half-inch object- 

 glass of 90° aperture. Mr. Thomas Ross sent me his first 

 object-glass of this new construction, for examination and 

 report ; and I believe, like many others, he hesitated to give 

 implicit credence to my account of its working. As he was 

 ignorant of the power of the " kettle-drum condenser,^' he 

 thought that the asserted resolution of that old microscopic 

 nebula, the P. angulatum, under so low a power as a half-inch, 

 even of large aperture, indicated the partiality of friendship 

 rather than the severity of honest criticism. Accordingly I 

 Was summoned before a microscopic jury, consisting of Messrs. 

 Leonard, Millar, Lobb, and Roper; and after sufficient and 

 careful examination, Mr. Leonard, as the judge, decided that 

 I might " take a rule nisi." 



As the half-inch and the condenser had not onlv not flinched 

 from any fair work, but had even trespassed on the domain of 

 the -ith and the ^th, I thought that I would show at last what 

 they could not do; and therefore, without the slightest expecta- 

 tion of taking anything for my pains, I placed on the stage of 

 the microscope a slide of the Amician test, the Navicula 

 rhomhoides, which was kindly presented to me by Mr. Powell, 

 whose fine TT^h, with its unequalled achromatic condenser, 

 reveals the exquisite skill which is bestowed on this almost 

 invisible work of the great Creator. It does one good, both 

 mentally and morally, to review such a work as this ; and, to 

 my astonishment and delight, I witnessed its resolution under 

 ray new arrangement. It is necessary, in this instance, to 

 use a deep eye-piece for attaining the requisite amplification ; 

 and as eye-pieces are instruments for measuring the imper- 



