M((Itiroo(Vs Finder Siipphmented. 71 



accidentally broken in Boston) ; tins fragment of the cover is so 

 mounted th;it part of the diatoms are in balsam and part of them 

 are dry. These I have no difficulty in resolving, counting, and 

 measuring, either dry or in balsam. I give here a Table of my 

 measurements, by which it will be seen that the Am. pellucida on 

 the Woodward slide is finer than that on Moller's " Probe Platte," 

 and I find it a little more difficult in balsam. 



Navicula crassinenns 78,000 lines to English inch. 



Nitschia curvula 82,000 „ „ 



Am. pellucida 87,0U0 ,, „ 



„ Woodward .. .. 92,000 „ „ 



I have seen no account of Am. p>ellucida having been resolved 

 and counted in balsam before. With lamplight and the above con- 

 denser I can resolve on J\[oller's " Probe Platte" up to and inclusive 

 Nitschia curvula, the Am. pellucida having thus far refused to 

 show its lines. 



The objective as I usually use it with the B eye-piece gives 

 about 1500 diameters : when I employ it in counting I use a B 

 p)ositive eye-piece and an achromatic concave lens in the draw- 

 tube (amplifier), which just doubles the power, giving me 3000 

 diameters. It bears this " eyepiecing " well, and by using a Tolles' 

 D solid eye-piece I can still see these fine lines at a power of 6000 

 diameters ; such objects as P. angidaium are well shown with this 

 power. 



I do not consider tliis objective one of Mr. Tolles' best resolving 

 objectives. He has made other objectives of both higher and lower 

 power that will excel this one in this respect. This objective is 

 quite as good by central light, has good working distance, adjusts 

 for covers up to yooth of an inch, in fact, according to Dr. Car- 

 penter's comparison, more of a "roadster" than " race-horse." 



I make this communication in order that English, Continental, 

 and American observers can compare the above results with their 

 own work. 



MrsEUM OF Comparative Zoology, 

 Cambridge, Mass., Dec. 25, 1871. 



VIII. — Maltwood's Finder Supplemented. By W. K Bridgman. 



Maltwood's Finder is unquestionably of considerable service to the 

 microscopist for his own individual use, but it requires the addition 

 of several adjuncts to render it generally useful when required in the 

 way of interchange. Thus, if a marked slide be sent to a friend at 

 a distance, the latter has had, hitherto, no means of ascertaining how 



