174 MEMORANDA. 



already made. The arrangements requires the upper edge of 

 the aperture in the brass plate of tl)e stage to be turned down 

 (in a lathe) so as to form a cell l-8th of an inch or more broad, 

 and of sufficient depth to allow of a slight shoulder on the 

 lower edge of the plate. Into the cell thus made a soft iron 

 ring, cut or turned out of sheet iron, is firndy pressed, being 

 so made as to fit tightly. If required, small adjusting screws 

 may be introduced beneath to regulate the requisite projection 

 of the soft iron ring above the surface of the stage. The 

 magnets are attached to the live-bux or ohject-liolder, and as the 

 surfaces of adhesion are by this plan large, they may be much 

 reduced in weight and more easily made, being punched or 

 cut out of soft sheet cast steel l-16th inch thick, and after- 

 wards hardened in the usual way. The subjoined drawing 

 shows the mode of attachment to the under surface of the 

 brass live-box or object-holder, and which may be effected either 

 by screws or solder, taking care that the opposing and dissi- 

 milar poles of the magnets are brought as near as possible 

 without contact. As magnets always lose power by frequent 

 breaking contact, this arrangement allows them to be remag- 

 netized without any disturbance of the stage of the microscope. 

 Both magnets and soft iron ring require to be ground flat. — 

 J. B. Spencer, 9, Kidbrooke Terrace, Blackheath. 



Curious effect of moisture on tiie markings of ihc Pleurosigiua. — 



In the Introduction to the ' Micrographic Dictionary,' now 

 publishing by Van Voorst, and of which Dr. Griffith and 

 Mr. Henfrey are the authors, it is stated, page xxxiii, " In the 

 valves of the more delicate Diatomaceae {Gyrosiyma, &c.), 

 the point is important tliat the line of fracture of the broken 

 valves passes through the rows of dots on the dark lines cor- 

 responding to them, showing that they are thinner and weaker 

 than the rest of the substance ; had these dots represented 

 elevations, the valves would have been stronger at these parts." 

 This appears very conclusive, but a phenomenon has recently 

 come imder my notice which, to my mind, is easily explicable 

 on the supposition that the dots are elevations ; not so, how- 

 ever, on tlie hypothesis that they are depressions. On a slide 

 containing the Pleurosiyma hippocampus, mounted d?-// by the 

 late Poulton (whose recent loss many microscopists will I am 

 sure deplore), there is one specimen of a Pleurosicpna which I 

 am unable to identify with any in the Rev. W. Smith's 

 Synopsis. Some moisture has gradually insinuated itself 

 between tlie thin glasses of the slide, and has almost entirely 

 obscured the markings which I used formerly to see most 

 beautifully as dots all over the surface of the shell. However, 



