138 CORRESPONDENCE. 



rectly you must diminish the aperture of the glasses to that of the 



eye." . . ■ 



As Dr. Pigott has quoted Sir David Brewster, it is to be presumed 

 he must be able to refer to some authority for his quotation, as it is 

 hard to believe that Sir David Brewster would maintain that different 

 areas of the surface of a lens corrected for spherical aberration would 

 give separate or non-concurring images, the essence of a corrected lens 

 or combination of lenses being that the whole area of its surface 

 concurs in forming every part of the image at the focus. The fact 

 just stated being demonstrable and well known, I presume it will be 

 accepted. 



But Dr. Pigott extends his doctrine to the improvement of the 

 microscope, and if his fundamental principle were sound it would be 

 equally applicable in both cases ; but the fallacy of separate areas giving 

 separate and different images of the same thing being demonstrable, it is 

 unnecessary to pursue that matter farther. Dr. Pigott, however, in sup- 

 port of his argument, gives a microscopical experiment, which is not 

 without interest. If a minute object be viewed under a glass of large 

 aperture, according to Dr. Pigott " a great many views of the object 

 will be produced, all different ; yet all these views are mixed up into 

 one compound image : the eye receives rays from every side of a 

 minute object at once : the observer looks at it from innumerable 

 points of view at one and the same instant : " and under these cir- 

 cumstances it is not to be doubted that very little could be seen. But, 

 says Dr. Pigott, reduce the aperture of the glass, and the object will 

 be seen in the greatest perfection ; and here, I am happy to say, I am 

 able so far to agree with him. 



The truth is, comparatively few objects are seen to advantage 

 under a large aperture ; in most cases one half of the pencils of light 

 cannot be brought into focus along with the other half, and the result 

 is frequently " fog " and indistinctness. 



With the large aperture the pencils have points of this shape 



==- ' F, and only such jmrts of the image will be 



seen as may be critically in focus, as at F ; while the pencils notin 

 focus cause indistinctness. With the contracted aperture the points of 



DE F 



the pencils are changed into this shape : |-j-| , 



which gives a greatly extended range, or depth of focus, almost equally 

 good at D, E, or F ; and features are thus distinctly seen at one 

 view which, with the large aperture, would be entirely out of focus, 

 and invisible. 



But this has nothing to do with the " separate images, all dif- 

 ferent," said to be formed by different areas of the microscopic object- 

 glass. 



G. S. CUNDELL, F.M.S. 



