970 Transactions of the Society. 



field, the greater or less intensity of the dark blue and violet has no 

 appreciable effect on the image, being drowned by the bright back- 

 ground produced by the other rays, and lamplight, containing as it 

 does a sufficient quantity of those blue rays which are predominant, 

 will not be inferior to sunlight as regards the effective wave-length. 



Even with sunlight, however, no greater number of lines can 

 be resolved than are showa in the original table for the line E. The 

 superiority of sunlight is based, not upon a different limit of the active 

 wave-length, but upon the fact that in consequence of the absolute 

 intensity of the light it is possible to utilize the full aperture of 

 the objective, which cannot be done with light of less intensity. 



When lamplight or daylight is used the relatively lower in- 

 tensity of the illumination renders it necessary to employ pencils 

 of perceptible breadth. The axes of the pencils will therefore 

 necessarily be at some distance from the margin of the objective's 

 aperture as shown in fig. 224 (A, dbrect light and A', diffraction 



Fig. 224. Fig. 225. 



pencil). The extent to which the axes are within the margin 

 represents so much lost aperture just as if it were reduced to the 

 dotted line. 



With sunlight, however, the intensity of the illumination is such 

 that an image may be obtained by using only a very narrow incident 

 pencil. The axis of such a pencil is practically at the margin of the 

 aperture, and the full aperture is therefore utilized (see fig. 225). 



This also shows that the figures given in the new table, 

 whether for white light, monochromatic light, or photography, will 

 only be attainable when the source of light is of great intensity; 

 otherwise there will be a loss of aperture. 



Although sunlight only has been referred to, it may be pointed 

 out that the electric (arc) light is little, if at all, inferior to sunlight 

 in regard to the utilization of the full aperture of the objective. 



Note hy Professor Ahbe. 



Prof. Abbe, to whom I sent a proof of the preceding paper for 

 his views as to continuing columns 3 and 4 to the lower numbers, 

 writes : — 



" There is one point as to which you may think that an ex- 

 planation is advisable. As you say, in the case of fig. 224 the 



