252 Transactions of the 



looking broad and flattened — I use the term " fit," because not 

 unfrequently I have seen the appearance, the tubercle displaced 

 from the base to which it has apparently belonged, and lying on 

 its side, a round hole in the flattened base strongly suggesting the 

 original locality of the small peg-like tubercle. In one specimen, 

 to which I can always refer by means of Maltwood's finder, a scale 

 has received some crushing, and several of these little " pegs " lie 

 about, looking as if they were knocked out of place ; two of them 

 appear reclining on the surface of the scale, and several more are 

 seen on the glass by its side. 



Although the Battledore scale is not strictly what would be 

 called an object for the polariscope, there is no doubt parts of its 

 structure, and particularly the tubercles and the ribs, do partially 

 fZe-polarize the light, and therefore polarized light becomes valuable 

 in examining the structure, and under a proper management of 

 the analyzer and the sized pencil of light transmitted, the tubercles 

 show with extreme brilliancy and reality, and moreover they seem 

 to be placed as I have represented, in shallow pits in the surface of 

 the scale in some specimens, while in others the ends of the pits 

 seem semi-continuous, as if a row of tubercles stood in a groove, 

 the sides of which were constricted at intervals. These pits may 

 really be elevations, and due to " inversions of the image," but 

 they do not look so, and I have put them down just as I saw them. 

 I am so afraid of being thought guilty of exaggeration, that I have 

 quite understated the eff'ects which can be got in these tubercles by 

 aid of polarized light. To employ this light to advantage, I prefer 

 daylight, with tourmaline and Nicol's prism only, the latter behind 

 the objective. For lamplight a pale-green ground from an appro- 

 priate selenite film seems an advantage. 



A word in concluding, to those who may wish to repeat these 

 observations on the markings of Battledore scales, or of kindred 

 structures. Scales are best seen by " transmitted " ordinary light, 

 when a " pin-hole " stop is placed like a small cap on the usual 

 wide-angled condenser, and by being very particular that both flame 

 of the lamp and object are in focus, or very nearly s'o, at the same 

 time. "When scales are looked at by " reflected " light, then I saw 

 them at their best by bringing up a little transmitted light at the 

 same time, such light being quite subordinate, and only for the 

 purpose of rendering the black shadows transparent. A similar 

 effect of course can be got by using a second lamp and " bull's-eye " 

 at the other side of the microscope, or even in a minor degree by a 

 bit of white card, placed in the stage beneath the object, so as to 

 reflect light, but on the whole I prefer the first form of " double 

 illumination " as equally satisfactory and far less troublesome. 



April 2Wi, 1872. 



