Minute Organic Particles. 19 



points teaches us to beware of creating them, and to manage effects 

 witli diffused light or clean shadows. 



(2.) Each particular ray of the spectrum has its own least circle 

 of confusion. 



The red rays which most obstinately refuse extinction form a 

 circle of least confusion farthest from the plano-convex lens of the 

 experiment. The violet rays form one nearest and the yellow 

 give a circle of least confusion midway. When achromatism is 

 made as perfect as possible, there is a residuary chromatic aber- 

 ration, chiefly consisting of greenish yellow on one side of the focus 

 and pale lavender on the other. But in this case the spherical aber- 

 ration is more visible in the best glasses than when the achromatism 

 is deranged in favour of a rose red or orange.* 



II. The nature of high-angled vision. 



All advanced physiological observers will agree with me as to 

 the comparative value of high angle and low angle in their micro- 

 scopical researches. Dr. Carpenter, F.E.8., so justly celebrated 

 for his attainments in this department of science, has incessantly 

 advocated the advantage of low angle. 



But there is a fashion in these things ; and so large a number 

 of people with microscopes make a toy of a noble instrument in 

 viewing diatoms and lines (in which it is well known, high angle 

 in a bad glass will often succeed when a low angle in a good glass 

 will fail), that a sort of rage for high angle among the buying 

 public has hardly yet subsided. 



When an irregular solid is viewed on all sides at once, a 

 confused image is necessarily presented. If it is a ridge we see 

 both sides at once ; the image of the left side is commingled with 

 that of the right : if one were blue and the other yellow, the 

 resultant image would be gi'een ; and if the two sides of the 

 minute ridge were different, one spotted and the other plain, both 

 sides would appear green and both sides spotted. If a minute 

 dice could be imagined as small, say as an organic particle of the 

 Podura, every side would appear to have the same number of dots 

 if all the sides could be seen at once, or the dice might appear 

 round and dotted all over. So an object-glass of high angle would 

 view the four sides all round at once from a thousand different 

 points of view. 



It is well known that for high angles it is the outer and not the 

 central parts of the glasses that do all the work ; and that it is 

 infinitely more easy to make an inferior high-angled objective for 

 diatom lines to be brought out by illumination tricks, than a low- 



* I defer the proof of this to another opportunity. I beg to state here, that I 

 continually find more and more value in " the colour test " I described in this 

 Journal for April, 1870, though at that time I confessed my ignorance of the 

 cause of the phenomenon. " Something recondite here beyond our ken." 



