18 On the Higli-Fower Definition of 



size. This disk, which might be called spurious, is merely the 

 diameter of the least circle of confusion. Its laws for a single lens 

 are well known.* 



I now substituted for the plano-convex stage lens one of the 

 finest glasses ever made for excellence of definition, and obtained a 

 disk which theoretically ought to have been the sixty-one thousandth 

 of an inch in diameter ; but upon accurately measuring it I found 

 it rather less than the sixteenth thousandth of an inch. Take the 

 ^o-u-o^th from the T^Foo^h, and the remainder, -sr^ooth, is the 

 diameter nearly of the resultant least circle of confusion. The 

 efiect of this is not in general seen, except with very brilliant light. 

 I say nothing here at present of the brilliant and gorgeous pheno- 

 mena displayed by " The cmcuLA.R solar spectrum " thus obtained 

 (already communicated to the Eoyal Society), nor of its revelations 

 (severely cruel) as to the errors of construction, achromatism, and 

 spherical aberration ; but this one thing I will venture to say, that 

 the circle of least confusion, in the unsuspected size of its diameter 

 is sufiicient to account for the great obstacles in the way, even at 

 present, of perfect definition ; and that a minute brilliant disk is 

 swelled out by it to nearly three times its jjrojjer size. There is an 

 example of this in the bright dots with which the Podura may be 

 bespattered by drawing out the tube a long way, which produces 

 for a corrected glass considerable over-correction. 



Again, if a brilliant refracting organic j)article be examined by 

 transmitted light, its natural point of greatest condensation of light 

 swells out, and it cannot be defined properly, on account of the 

 bright ring or halo which surrounds it. Sometimes this result can 

 be got rid of by using such tricks of side illumination as destroy 

 this efiect. Also, if exceedingly high powers be used, such as the 

 one-fiftieth immersion, the focal depth is so shallow that the 

 plane of vision may pass through the centre of an organic particle 

 {a monad suppose) and be below the point of greatest light con- 

 densation.! I had a fine view of a monad the other day, which 

 appeared to rotate and twist about, but a bright halo surrounded it, 

 probably indicating the place of the cilia, which I could not see. 

 The monad appeared hlue, and there was a heap of them looking 

 rather quiet but extraordinarily well defined. 



The result of this efiect of the circle of least confusion for bright 



* Dr. Parkinson, F.R.S., ' Optics,' p. 57, has this explanation : — " Since tlie 



circle of least confusion is the nearest approach to a point When an object 



of finite size is viewed by reflexion or refraction, we may consider the visible 



image to be the locus of the circles of least confusion These circles will 



overlap each other, and the image consequently will be more or less confused." 



" We may regard the comparative size of the circles of least confusion in 

 different cases as a measure of the comparative indistinctness of the visible 

 image." 



•j- In refracting sphericular particles in a less dense medium, this point or 

 focus lies above it, and vice versa in a denser medium below it. 



