LIMITS OP OPTICAL CIPICITY OF THE MICROSCOPE. 433 



kind reversed. The readiest mode of finding whether the illumina- 

 ting apparatus gives sufficiently wide pencils of light is to examine 

 the ocular image with a magnifying lens after the instrument has 

 been focussed. 



1 must now relate here the failure of an attempted improvement, 

 the negative result of which is significant. I thought myself 

 justified in inferring theoretically, that the difi'raction of the 

 microscope might be neutralised if the points of the narrow 

 aperture which causes this diffraction were made singly and 

 separately luminous, and that this could be aff'ected by causing 

 a sharply defined optical image of the source of light, {e.g., sun 

 illumined cloud,) to be thrown by a lens on the plane of this 

 aperture. Years ago I tried experiments of this kind on a 

 Nobert microscope, provided with immersion lenses, giving 

 excellent definition. The result of this trial shewed that it 

 was perfectly indifi^erent whether the image of the source of light 

 fell on the plane of the object, or of the objective. The difi'raction 

 fringes caused by the use of a very deep ocular remained un- 

 corrected. More recently I have convinced myself by fresh trials 

 made with larger lenses, that such a procedure is useless. When 

 a good achromatic lens of about 18 inches focus, is so placed as 

 to show a sharp image of the source of light, (as in this case a 

 bright sky cloud,) upon the surface of a system of lines scratched 

 on glass, the images of many separate luminous points will be 

 thrown upon the variously transparent clefts of this grating, and 

 it might be supposed that the interference of rays which had passed 

 through neighbouring clefts would cease. If however we look 

 through the grating towards the lens, and place before the 

 lens pieces of card pierced with fine slits, we see with the naked 

 eye just the same diffraction fringes, as well at these slits as at 

 the outer edges of the cards, as would be seen if the lens were 

 removed, or the grating set out of focus. 



Instead of the lines I then made trial of two fine linear slits 

 cut in cardboad, with an interspace of about one m.m. and through 

 which I could see with the naked eye a system of very fine 



