2 Transactions of the 



makes with the body of the instrument, will be found a very con- 

 venient one, as the stage of the microscope will be horizontal, 

 while the telescope will be inclined at a comfortable angle for the 

 eye. It is obvious that more light passes through the slit of this 

 spectroscope, because it is so much nearer the object-glass, and also 

 that more light passes through the prisms, because they are fewer, 

 and the collecting lens being done away with, the light has only 

 one instead of two lenses to traverse. All this gives a greatly 

 increased brilliancy to the spectrum, and enables objectives of high 

 power to be used, as also a telescope. When it is required to 

 examine the spectra of very minute objects, it is important to 

 ascertain that the whole of the light which is used to form the 

 spectrum passes through the substance which is being examined. 

 In order to make certain that this is the case, the tube M 

 (carrying the collimating lens and the spectroscopic apparatus and 

 sliding in the outer tube C) should be removed, leaving the outer 

 tube C carrying the slit and all its adjustments fixed to the micro- 

 scope. The draw-tube of the microscope having the erector screwed 

 into its place at the lower end of it, should now be inserted into the 

 outer tube C, and an eye-piece should be placed in the usual 

 position in the draw-tube ; the open slit can now be easily and 

 clearly focussed by sliding the draw-tube in and out until the slit is 

 seen distinctly. The object on the stage of the microscope being 

 now placed in focus in the usual way, both it and the slit will be in 

 the same field of view, and it will therefore be easy to make certain 

 that the material under examination occupies the proper position 

 between the jaws of the slit. 



Diagram No. 2 shows this arrangement. M' is the draw-tube 

 of the microscope ; 0, the erector ; and P one of the negative eye- 

 pieces of the microscope. The draw-tube being removed, the 

 inner tube M carrying the spectroscope can now be replaced, and 

 the observer will be certain that the slit corresponds to the proper 

 radiant point. 



II. — On an Aerial Stage Micrometer: an improved form of 

 engraved " Lens-Micrometer " fo'r Huijghenian Eyepieces, 

 and on finding Micro metrically the Focal Length of Eye-pieces 

 and Objectives. By G. W. Koyston-Pigott, M.A., M.D., &c. 



(Read before the Royal Microscopical Society, Dec. 4, 1872.) 



The writer wished to draw attention to a mode of using micro- 

 meters, which he thought was somewhat novel. He has found very 

 great convenience from using an aerial image of micrometer spider- 

 lines. The micrometer which he employed was one of Browning's 



