104 MEMORANDA. 



can give is this : The " gauge-taps" issued by the Micro- 

 scopical Society are slightly taper and differing in size, con- 

 sequently, when they are sent out to the makers, there are 

 directions with them, as to how far each one is to screw up 

 the nose-piece of the microscope, and that if the screw is made 

 only "125 in length, it may prevent some of the taps from 

 screwing up as far as they should. On my own part, 1 see 

 no objection (if proper means he employed) to the maker 

 cutting the screw in whichever way he pleases. I perfectly 

 well remember pointing out to Mr. Brooke the assistance it 

 would be to the workman when cutting the inside screw 

 by hand, to make it a greater length than *125, but, if the 

 work be done in a machine, this difficulty ceases; and it is 

 important to save the wear of the screw-tool as much as 

 possible. 



And uoav, in conclusion, may I also tell what I know as to 

 how " the plan recommended by the Microscopical Society 

 has been found to work well amongst those who have adopted 

 it." Of the three houses besides our own who arc vising the 

 " universal screw," one has ordered a set of the cutting- 

 gauges. Another firm are making their outside screw con- 

 siderably smaller than Whitworth's gauge, and their inside 

 screw to the Society's tap. The other firm are also using 

 the Society's tap, but they are making their outside screws 

 considerably larger at the bottom than Whitworth's gauge, 

 and it is this firm, so far as I can ascertain, Avho have altered 

 the nose-pieces of our microscopes, but, in one instance which 

 has come to my knowledge, after having made the alteration, 

 an object-glass which they sent subsequently, proved too 

 large, and would not screw into the microscope at all. — 

 Richard Beck, Upper llolloway. 



Harrison on a new Pleurosigma. — Somewhat better than 

 twelve months ago I was visiting a friend in Lincolnshire 

 ( W. Parker, Esq., of Thornton House), and he showed me a 

 slide containing a few frustules of a Pleurosigma which I 

 had not before seen; I enquired where lie bad obtained them, 

 and he went with me to the drain (a run of fresh water), but 

 we could not find any at that time, I was paying another 

 visit, to the same friend, about three weeks ago, and we pro- 

 cured a pretty good gathering of them, and as 1 could not 

 find any account of it in any previous work, 1 have named 

 it Pleurosigma Parkerii. It is about the same size as the P. 

 J'asc'ii/la, but broader in proportion to the length, and the 

 ends not so much contracted. 



