178 



EARDWICKE'S SCIENCE-GOSSIP. 



[Aug. 1, 1870. 



mounted, and, to my notion, is better without the 

 use of a selenite. Everything depends upon the 

 " setting " of the analyzer prism, which must bear 



Fig. l6g. Sepiostaire, Ground Plan. 



a certain relation to the arrangement of the sepio- 

 staire. This found by experiment, the polarizer 

 may be rotated, and a wonderful play of colour will 

 be the result. 



Fig. 170. 

 Sepiostaire, canaliculi 

 of basal membrane. 



Fig. 171. Sepiostaire, Horny 

 Case of, x 100. 



A little diluted acid will decalcify the lamina, but 

 leaves only a little almost structureless membrane, 

 having nothing particular noticeable about it, 

 though certain evidence of striation in the vertical 

 lamina? would lead one to expect variations of struc- 

 ture in the membrane itself, especially as the 

 "pillars" arc but partially calcified. 



The homy outer layer, or "shell," differs from the 

 " bone " proper, and is likened by Dr. Carpenter to 

 the " hinge-tooth of Mya," from which, however, it 

 has'points of difference, which our cut (fig. 171) 

 attempts to bring out as strongly as possible. 



H. PfJCKLINGTON. 



Zetjzera jEsculi (Wood-Leopard).— This moth 

 abounds in some gardens here, and has completely 

 destroyed several fine ash-trees. I found the moths, 

 male and female, on the trunks of the trees, in the 

 afternoon, just out of the chrysalis ; they are very 

 sluggish, and come off on the edge of the net when 

 touched with it.— iZ. C. Leslie, Erith. 



DIATOM MARKINGS. 



A LL microscopic students must feel grateful 

 ■*-*■ to the Rev. J. B. Reade for his demon- 

 stration of the true markings of Diatoms, and 

 his description of the method of observing them 

 by means of the prism and parallel unilateral 

 light, as given in his very interesting paper in the 

 Popular Science Review for April last. The rotating 

 stage confirms his discovery, and makes us sure 

 that our miscroscopes no longer mislead us. It is 

 humiliating, that so many old observers (ourselves 

 among the number) should have been so long "led 

 by the nose," to see dutifully just what they were told. 

 They must have seen the hemispherical markings, 

 and they were even photographed by Dr. Maddox, 

 as shown in the beautiful frontispiece to Dr. Beales' 

 " How to work with the Microscope," but they 

 were 'not recognized as true. Now that we know 

 what to look for, and how to look for it, other me- 

 thods may be used; and we may venture to point 

 out the use of Wenham's Parabolic Illuminator 

 for this purpose. Stop out the half furthest from 

 the light, and with the concave mirror a little 

 diverted towards the source of light (a clear 

 sky is the best), throw a spot of light full 

 upon the open part. The illuminator must be 

 withdrawn from a quarter to half an inch be- 

 low the stage, according to the power used. A 

 strong ray of light will then fall obliquely on the 

 object, and will be rendered virtually parallel by 

 the convexity of the illuminator. With a one-eighth 

 power and third eyepiece, giving, with the draw- 

 tube, a diameter of 2,000 linear, the hemispherical 

 markings of Pleurosigma angulahmi are in this way 

 beautifully shown. Navicula gracilis (the Amician 

 test) is resolved into rows of hemispherical dots 

 set very close together; and the lines of Gram- 

 matophora subtilissima, of which we were once 

 glad to obtain a glimpse, are similarly resolved. 

 Pleurosigma rhomboides, under a very favourable 

 light, can be scarcely " glimpsed." It demands a 

 much higher power to see that exquisite diatom 

 clearly. 



We have not yet invested in a one-sixteenth 

 immersion lens, preferring to wait for further 

 light on "aberrations" and "clearers." It may 

 be worth mentioning that the Navicula, gracilis 

 and rhomboides, are both abundant in the Lapland 

 Bergmehl, which is to be found in most cabinets. 

 The Wenham Illuminator may be kept one-half 

 stopped off even when used for other objects. It 

 will, by unilateral light, bring out some structures 

 not otherwise easily seen. In working with the 

 higher powers by very oblique light, a liberal use 

 must be made of the adjustment-screw. 



S. S. 



