340 SUMMARY OF CITRRENT RESEARCHES RELATING TO 



shine up on a dark background. By slowly uncrossing the Nicola they 

 are easily located in the section. 



Polariscope.*— ^E. M. Nelson states that : — " It is not generally 

 known that diatoms are polariscope objects, for it is understood that 

 silex will not polarize, so probably diatomists, accepting that statement, 

 have not further troubled themselves about the matter ; at least, that is 

 what I have myself done, and thought that, with the exception of a 

 Flevrosigma balticum, few diatoms were suitable objects for a polariscope. 

 This, however, is quite a mistake, for the other day I observed that dia- 

 toms with thick silex, such as C'oscinodisci, Aulacodisci, the polygonal 

 structure of the larger Triceratia, the thick radiating bars of Arachnoi- 

 disci, the edges of NavicuJa Ujra, &c., exhibited polarizing effects 

 strongly. These diatoms were all mounted dry on cover. The very 

 thick diatom Eupodiscus argus shows these effects even in a balsam 

 mount. An Aidiscus oamaruensis Gr. and S., when illuminated on a 

 dark ground with a condenser and stop, ^ objective of 5 N.A., with 

 polarized light (blue), gave one of the most charming images of a diatom 

 I have ever seen. Polycistines, which are also silicious, showed no trace 

 of any polarizing effect. Polycistines are animals, and it may be that 

 silex deposited by animals does not exhibit polariscope effects, while that 

 deposited by plants, such as diatoms, will do so. Foraminifera are 

 mostly lime, and show splendid colours under polarized light. It is sur- 

 prising that they are not more often examined that way, as it might be 

 ■of assistance in determining species. Globigerinse show the black cross 

 plainly. As few microscopists, who are not mineralogists, understand 

 how to work with polarized light to the best advantage with an ordinary 

 microscope, and as no help is given in any of the text-books, the follow- 

 ing notes may be of assistance. Set up the microscope with substage 

 condenser in the ordinary way. Place the polarizer below the substage, 

 and the analyzer above the eyepiece. Rotate the analyzer until the field 

 becomes dark, and note its position for a fiducial point. Rotate the 

 analyzer to 90° on either side of this, and place a f wave-plate selenite 

 in the stop-holder below the condenser, and rotate it until a fine back- 

 f^round is obtained. Note : — If the selenite is placed in a rotary 

 holder above the substage condenser a smaller one would do. This 

 forms the best polarized field upon which to view objects. As a Nicol 

 is now an expensive luxury, an excellent polarizer can be made by placing 

 eleven disks of extra thin sHp glass, each having three little spots of thick 

 notepaper gummed to its periphery, to keep the glasses from contact. 

 Below the last piece of glass a piece of black (needle) paper should be 

 placed. These glasses should be kept quite clean, the mirror cell should 

 have a screw ring, so that they may be easily removed. As a concave 

 mirror is never used, they may quite well take its place. The cost of this 

 mirror cell and the eleven glasses would be a trifle, compared with that 

 of a large Nicol. To set the mirror in the correct position, cut an angle 

 of 114° out of a postcard, bisect this angle by a line ; place the angle on 

 the mirror with one arm of the angle pointing up the optic axis of the 

 microscope, then the other arm will point to where the light ought to 



* English Mechanic, cv. (1917) p. 184. 



