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only a low degree of sensibility. The method requires hardly any 

 apparatus ; it is extremely easy in execution ; and it has the great 

 advantage of rendering the observer independent of sun-light. On 

 these accounts the author conceives that it might be immediately 

 applied by chemists to the discrimination between different sub- 

 stances. The method is as follows : — 



A large hole, which ought to be several inches in diameter, cut in 

 the window-shutter of a darkened room, serves to introduce the light, 

 and a small shelf, blackened on the top, attached to the shutter im- 

 mediately underneath the hole, serves to support the objects to be 

 examined, as well as one or two absorbing media. The hole is 

 covered by an absorbing medium, called by the author the principal 

 absorbent, which is so selected as to let through, as far as may be, 

 the feebly illuminating rays of high refrangibility, as well as the in- 

 visible rays still more refrangible, but to stop the rays belonging to 

 the greater part of the visible spectrum. A second medium, called 

 by the author the complementary absorbent, is chosen so as to be as 

 far as possible transparent with regard t»o those rays which the first 

 medium stops, and opake with regard to those which it lets through. 

 The object to be examined is placed on the shelf, and viewed through 

 the second medium. If the media be well-selected, they together 

 produce a very fair approach towards perfect darkness ; and if the 

 object appears unduly luminous, that arises in all probability from 

 " fluorescence." To determine whether the illumination be really 

 due to that cause, it is commonly sufficient to remove the comple- 

 mentary absorbent from before the eyes to the front of the hole, 

 when the illumination, if it be really due to fluorescence, almost 

 wholly disappears ; whereas, if it be due merely to scattered light 

 which is able to get through both media, it necessarily remains the 

 same as before. In the case of objects which are only feebly fluo- 

 rescent, it is sometimes better to leave the second medium in its 

 place, and use a third medium, called by the author the transfer 

 medium, which is placed alternately in the path of the rays _^incident 

 on the object-end of the rays coming from it to the eyes. 



Independently of illumination, the change of colour corresponding 

 to the change of refrangibility, and the difl'erence of colour with 

 which the object appears, according as the transfer medium, or the 

 complementary absorbent used as a transfer medium, is held in front 

 of the eyes or in front of the hole, afi^ord in most cases a ready mode 

 of detecting fluorescence. 



Instead of trusting to the absolute appearance of the object, it is 

 commonly better to compare it with some fixed standard. The 

 standard substance ought to be such as to scatter freely visible rays 

 of all refrangibilities, but not to give out rays of one refrangibility 

 when influenced by rays of another. The author employed a white 

 porcelain tablet as such a standard ; and the object to be observed 

 was placed on the tablet, instead of being laid directly on the black- 

 ened shelf. 



Another mode of observation consists in using a prism in combi- 

 nation with the principal absorbent. The object being placed on 



