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single microscope in its various forms of single lenses, doublets, 

 jewels, and triplets ; it is therefore from personal experience I have 

 referred to the fatigue and to the permanent injury of sight which 

 they produce. My most constant object has been to improve, if I 

 could, the preparation of the object for vision, especially what is 

 technically called its illumination. 



" When I commenced, the only mode of illumination in use 

 was the light of the sky by day, or of a candle or lamp by night, 

 reflected on the back of the object, and occasionally condensed by 

 a lens, but the quantity of colour thus produced rendered this 

 far from satisfactory. When Pritchard introduced his exceedingly 

 commodious form of achromatic microscope, of one of which I soon 

 became possessed, he gave the means of applying the direct light 

 of a candle to the back of the object without the intervention of 

 any mirror or condensing lens. This, besides being so exceedingly 

 convenient and so free from trouble, was so great an improvement on 

 the previous mode, that for a long time I rested satisfied with it. 

 Meanwhile the improvements in achromatic object-glasses continued 

 to progress, slowly, it is true, at first, still, very decidedly, and as 

 from time to time objectives of larger aperture were made, the 

 simple illumination became less and less efficient ; recourse was 

 then had to the so-called achromatic illumination : that is, forming 

 by means of an achromatic lens an image of the luminary coinci- 

 dent with the object under examination ; this acted very satisfac- 

 torily with some objects, but not so much so with others, but the 

 image of the source of light always mingled inconveniently with 

 the object under examination. Then came Read's dark ground illu- 

 mination, in which, by means of a very oblique pencil of light, 

 the object was shown luminous on the dark field of the instrument. 

 It would however be tedious and useless to follow up minutely these 

 slow advances ; it is sufficient for my purpose to say that such as I 

 have described was the state of the matter when I began to exa- 

 mine the subject. 



" It appeared to me that, in transmitting light through a trans- 

 lucent object under the microscope, the image does not reach the 

 eye by reason of the interception or coloration, by the parts of the 

 object, of the light diverging from its original source; but that in 



