186 COBRESPONDENCE. 



scopic object to bo the head, in relief, of the Venus de Medicis, on a 

 much smaller scale than the beautiful microscopic portraits of Mr, 

 Dancer, of Manchester, and that the microscopical observer is re- 

 quested to make a drawing of it. We cannot venture to say what 

 would be its expression, but we are sure that it could have no resem- 

 blance to the original, both ears being fully brought out and almost 

 the v/hole round of the head. In like manner every ridge in a micro- 

 scopic object will show to the observer both its perpendicular sides as 

 well as the side opposite the eye, and the resulting picture will be a 

 coincident combination of a thousand different pictiu-es, as seen from 

 every point of the object-glass. The reason is therefore obvious why 

 a large aperture shows lines that are invisible with a small ajjerture. 

 The relief of a bust, or of a relievo either basso or alto, is best seen 

 when we look at it in profile. Its height is then actually seen, whereas 

 it is merely inferred when we look it full in the face. When the 

 raised lines of test-objects ai*e fully illuminated only obliquely, they 

 are seen obliquely, and consequently much better than with a small 

 aperture, which may not show them at all ; not because the object- 

 glass is inferior in penetrating * power, but because the thing looked 

 at in one case is not the thing looked at in the other, and is actually a 

 smaller object." 



Hence the perfection of a microscope consists in its having the 

 "smallest angular aperture consistent with distinct vision. Such a 

 microscope will not show certain objects of great minuteness, but it 

 will give a perfect representation of what it does show. The large 

 angular aperture will show the same objects and others more minute, 

 but whatever it does show will be a mockery of the truth." 



Mr. Cundel will find further remarks by Sir David Brewster on 

 the necessity of small apertures for truthful portraits, in the earlier 

 reports of the papers of the British Association. 



I am. Sir, your obedient servant, 



G. W. EoYSTON-PlGOTT. 



Dobs Cliona Burrow? 



To the Editor of the. ' Monthly Microscopical Journal.'' 



Sir, — In your last number you were good enough to notice a 

 paper of mine published in the ' Transactions of the Devonshire 

 Association for the Advancement of Science, Literature, and Art,' 

 1871, " On the Boring of Mollusca, Annelids, and Sponges, into Wood, 

 Eocks, and Shells." 



At the conclusion of your remarks you say, "unfortunately for 

 Mr. Parfitt," in allusion to a paper published by Mr. Waller in the 

 ' Journal of the Quekett Microscopical Club.' Now, Sir, this exclama- 

 tion of yours rather alarmed me ; I thought I had either fallen into 

 a tremendous error, or that Mr. Waller had discovered the whole 



* This in modem language has been converted into resolving power, whilst 

 penetration refers to depth of fucus. — G. W. K. P. 



