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LXX. On the Fitting-up of Microscopes for the Examination 

 of Opaque Objects requiring high powers ; and on the Con- 

 struction of a Focimeter. By Sir. G. Dakin. 



To the Editors of the Philosophical Magazine and Annals. 



Gentlemen, 

 A S microscopes are now fitted up by the opticians, a nu- 

 ■**■ merous and beautiful class of objects is entirely lost to ob- 

 servers ; viz. those opaque objects which require a high power. 

 My single microscope, which I had purposely fitted up with 

 the highest powers both opaque and transparent, is almost 

 useless for this purpose, as the highest opaque eye-piece is 

 l-6th of an inch focus. I have seen the scales of the diamond- 

 beetle as an opaque object through a lens which I think Dr. 

 Goring said was l-60th of an inch focus. He has certainly car- 

 ried it quite to the maximum, as there was rather a want of light 

 and distinctness, which I attribute in a great measure to the 

 best part of the speculum being lost by the introduction of 

 the deep convex cup which holds the lens ; nevertheless the 

 beautiful lines on the scales were plainly to be seen, even with 

 this high power. I have made the whole of my transparent eye- 

 pieces answer as opaque ones, in which the lenses are placed 

 on the outsides of the speculums ; consequently the whole of 

 the central and best part of the speculum is brought into 

 action. Opticians seem to forget that the light which is thrown 

 on the speculum is already condensed, and that such large 

 ones are not necessary, unless il be to compensate for the 

 clumsy manner in which they generally fit up the opaque 

 slides. Objects of this sort ought to be fixed on small cylin- 

 ders of jet or ebony, and these glued on slips of glass: by this 

 means little or no light is lost. The speculums I made were 

 about £, § , £, £, and Jth of an inch diameter : they should 

 be fitted into a brass ring (fig. 1.), to which should be soldered 

 a piece of wire, fitting a hole in a larger piece of wire (fig. 2.), 

 which should slide easily through the stage (fig. 3.) ; or they 

 may be laid on the slips of glass over the objects (fig. 4.), provided 

 the cylinders are adjusted to bring the objects into the focus of 

 the speculums. When the highest powers are used, take a very 

 small piece of the object and lay it on a small cylinder, which 

 must have a fine* hole in the side, for the purpose of fixing it 

 on the point of a fine needle ; place the object in the centre of 

 the stage, and bring the speculum down so as to illuminate 

 the object as much as possible. The eye-piece must then be 

 brought down close to the hole in the speculum, the eye being 

 now placed in its right position. The eye-piece may be raised 



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