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372 B. T. LOWNE ON INTERFERENCE PHENOMENA. 



With objectives of large numerical aperture the working dis- 

 tance is short, and with a large pencil much light is reflected upon 

 the object. With immersion lenses the reflection from the cover 

 lass and the front of the objective is practically done away with, 

 so that all the light reflected from the upper face of the front lens 

 falls upon the object. 



Five per cent, of the light which falls normally on the back sur- 

 face of a glass lens is reflected, whilst the quantity which is reflected 

 by oblique incidence rapidly increases; much light is totally 

 reflected, the whole converges after reflection once, twice, or thrice 

 towards the object, and it must be remembered that only the centre 

 of the pencil falling upon the back surface of the front lens is 

 transmitted to the eye, whilst the whole pencil is concerned in the 

 illumination of the object from above. 



I believe that this is the great advantage derived from high 

 angles of aperture, and more especially from immersion objectives. 

 The elimination of the false diffraction images resulting from the 

 large illuminating pencil and the reflection of light from the object 

 appear to me to be the causes of the great increase of definition 

 attained by their use. The view propounded by Professor Abbe 

 that they collect outlying diffraction pencils appears to me quite 

 inadequate to explain the increase of definition. 



[Since I read the above paper I have made numerous experiments 

 with my oil-immersion lens, to determine as far as possible the 

 illuminating power of the reflection from the back of the front lens. 



I have mounted a number of objects which are known to be quite 

 opaque, such as oxides of metals in fine powder or granules, crystals 

 of silver, etc. These objects are seen to be quite black, or nearly 

 black, with air lenses of low angle, but are brightly illuminated from 

 above with an oil lens. Thus tin oxide has the appearance of white 

 spar illuminated by the direct rays of the sun. The black crystals 

 of silver are frequently intermixed with white crystals with a metallic 

 lustre, these are seen on the surface of large black masses most 

 brilliantly. The globules of mercury already alluded to give a 

 bright central image of the flame of the lamp, and appear as 

 metallic globes when the illumination is properly adjusted. I am 

 still engaged in these experiments, and will bring the matter before 

 the Club as soon as my experiments are concluded.] 



