430 Mr. Dakin on the Microscopical Examination 



to its true focal distance; this will prevent the lenses being 

 injured by coming in contact with the speculum. 



The highest power in my microscope is about l-46th 

 inch focus ; but a lens of l-30th inch focus may be used by a 

 novice with ease and very great advantage, and the object 

 will be as well illuminated as by one of the large speculums. 

 As they are very easily made, I shall describe the method of 

 making a l-4th inch one. Melt a little fine silver by the blow- 

 pipe into a globule about 1-lOth of an inch diameter ; hammer 

 it out till it is full l-4th inch wide, and as thick as stout fools- 

 cap paper : take a piece of brass wire, barely l-4th inch thick, 

 and file one end of it hemispherical ly ; then lay the silver on a 

 piece of lead about l-4th inch thick. Hollow the silver to 

 the shape of the wire, and then drive it quite through the lead, 

 turning the wire at the same time : make a small hole in the 

 centre, and fix it on the end of a stick of sealing-wax ; then file 

 a piece of slate-pencil, barely l-4th inch diameter, to fit the silver 

 cup, and grind them together with fine emery, till it is of a uni- 

 form figure quite over the surface ; then wash the emery off, 

 and grind the scratches out with the pencil, washing the mud 

 away with clear water. To polish it, cover the pencil with thin 

 silk, rub a little tallow and jeweller's rouge on the silk, and 

 work them together till the speculum is beautifully polished. 



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I strongly suspect that the focal distances of lenses are ge- 

 nerally 



