148 Transactions of the 



instrument described by me is similar in principle to the original 

 form of the spectroscope applied to the microscope proposed by 

 Dr. W. Huggins in a paper read by him before the Microscopical 

 Society on the 10th May, 1865, and described and figured in the 

 ' Microscopical Journal ' for that month, entitled " Note on the 

 Prismatic Examination of Microscopic Objects." I regret that I 

 did not know of this paper by Dr. Huggins before reading my 

 own, but a reference to it will show that the dispersive power 

 employed was that obtained by the use of two prisms. There can 

 be no doubt that the amount of dispersive power which will be the 

 best for one kind of object, will not be always the most suitable for 

 all other objects, and that therefore sometimes one and sometimes 

 two prisms will give the best results. Much will depend upon the 

 amount of light available, and it would therefore be useful to so 

 arrange matters that either one or both prisms could be used at 

 pleasure, but as there is plenty of light available when the slit is 

 placed sufficiently close to the object-glass, and the collimating lens 

 is of adequate angular aperture, two prisms, under ordinary circum- 

 stances, can easily be used without rendering the absorption bands 

 at all undefined or indistinct. Dr. W. Huggins doubtless places 

 his slit at a distance of three or four inches from the object-glass, 

 because it gives a larger image of the object relatively to the slit, 

 and because the angle of divergence is greater the nearer the image 

 or slit is to the object-glass. The maximum of light (irrespective 

 of the size of the object) will be when the angle of the collimator 

 is equal to the angle of divergence. Now, when a very accurate 

 inspection of the minute portions of small objects is required, 

 the position of the slit must accurately correspond to the place 

 where the image of the object on the stage is formed behind 

 the object-glass ; and as this position of the image can be made 

 either to approach or to recede from the object-glass, at the will of 

 the operator, by merely altering the distance between the object on 

 the stage and the object-glass, by means of the usual focussing 

 arrangements, almost any position of the slit can be easily made to 

 be in the conjugate focus behind the object-glass ; and as the 

 nearer the slit and image are to the object-glass the farther the 

 object-glass must be removed from the object, the motion is in a 

 possible direction ; and as the image diminishes as it approaches 

 the object-glass, its brightness rapidly increases, and therefore the 

 closer the slit is to the object-glass, the more the light will be in- 

 creased which is received within the jaws of the slit ; but the closer 

 the image is to the objective the larger will be the angle of diverg- 

 ence, and therefore the diameter of the collimating lens must be 

 increased in proportion, and its focal distance lessened. 



Suitable proportions appear to be reached when the slit is 

 placed at a distance of 1^ inch from the objective, and the colli- 



