Minute Organic Particles. 21 



have to regret that my residence in the North at that time 

 prevented me superintending the preparation of the Plate. The 

 drawing then taken by a lady of talent, unaccustomed to the micro- 

 scope, was everything I could desire. Mr. Eeade, our former Pre- 

 sident, however substituted, by some mistake I presume, a rough 

 sketch of mine for the exquisite drawing sent to him as made by 

 the lady. I endeavour to rectify that mistake by the accompanying 

 Plate. 



III. The confusion of images of a mass of refracting particles 

 huddled together, in known or unknown forms, renders their 

 proper definition severely difficult. 



This appears at present to be " the pons asinorum " of optics, 

 to which " the pons " of Euchd is a trifle.* It is of no use for 

 people to try and prove a negative. A says he cannot see a certain 

 structure. A therefore says it cannot exist. The absurdity of 

 such reasoning would equal that of a blind man who swears there 

 cannot be such a thing as colour, because he has never seen it. 



I have shown (' Phil. Tr.,' 1870) that if the secondary image of 

 a given structure be examined above or below the best focus, it 

 often appears quite as real, though totally different, as the thing 

 itself. Suppose there are two transparent structures, and the false 

 images of the upper happen to coincide with the real image of the 

 true ; one of two things happens ; either both are obliterated or a 

 mirage is presented. More than this, if the object-glass happen to 

 be over-corrected spherically, by the observer, then the false images 

 of the lower mingle with the true images of the upper. 



Conversely, if the glass happen to be under-corrected spherically, 

 the false images of the ujjper structure alone mingle with the true 

 images of the lower.f 



I have no doubt this is the reason of Dr. Groring stating that 

 of the two he " i^referred an under-corrected glass" because this 

 throws the false images in a lower plane more out of focus. Ac- 

 cordingly it will be found that in complex structures a glass under- 

 corrected spherically will be better able to define the upper surface 

 than one over-corrected. 



I Y. The nature of mixed shadows. 



It is not long since objectives (at that time thought to be per- 

 fection) could only resolve difficult tests by using two obliquely illu- 

 minating lamps at once, or two sets of rays at right angles to each 

 other : this caused two shadows, each set forming its own par- 

 ticular lines. The Ehomboides used to be shown, after anxious 

 labour in this way: subsequently two slots were cut in a stop 

 in a wide-angled achromatic condenser to produce similar effects. 

 No one can manage the most dehcate definition who disregards 



* The fifth proposition of Euclid has long received this name. 

 t ' Phil. Tr.,' vol. ii., 1870. 



