ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 721 



consequently, balsam let fall on them would bo almost certain to 

 disperse them. 



In most cases it is advantageous to mount the diatoms on the 

 cover, which is easily done by first fastening it to a slide with a drop 

 of glycerine, which will not evaporate during the process of mounting, 

 and is easily removed afterwards. Large diatoms, such as Arachnoi- 

 discus, when mounted on the slide and examined by reflected light, 

 are apt to show a slight haze surrounding the group, instead of the 

 intense black ground which should be presented when all light is 

 shut off from below the stage. This is caused by reflection from 

 the under surface of the slide, and can be avoided by mounting on 

 the cover and placing some dead-black material at the bottom of 

 the cell. 



If Polycistina or Foraminifera are to be mounted, a thicker layer 

 of gum should be placed on the slide than for diatoms, as these 

 objects, from their peculiar forms, have usually a very small part of 

 their surface in contact with the slide. 



The author considers "this branch of microscopic art as quite 

 legitimate " where selected species have to be mounted and provided 

 scientific value is not sacrificed to mere prettiness. He also says that 

 he has recently used the gum process with all balsam-mounted diatoms 

 even when they are not arranged symmetrically for the sake of the 

 security it affords against the valves being displaced by slight 

 pressure on the cover-glass, or by the slide being kept in other than 

 a horizontal position, also for the advantage of being able to mount 

 the valves in different positions so often necessary in order to get an 

 exact idea of their true form. 



Kain's and Sidle' s Mechanical Fingers.* — Mr. C. H. Kain de- 

 scribes a simple mechanical finger for use with any Microscope that 

 has the fine adjustment on the nose-piece. It is designed to obviate 

 the inconvenience of the one described by Professor H. L. Smith, f 

 which requires the loosening and tightening of the objective for the 

 purpose of focussing. 



It consists essentially of a slotted bar (Fig. 140), which may be firmly 

 clamped to the upper (immovable) bar of the fine adjustment by means 

 of a milled-headed screw. Through the end of this is fastened a 

 round rod, at such a distance from the objective that, when lowered, 

 the end will not strike the stage. Over this rod slips a split tube, to 

 which is soldered, at an angle, a smaller tube. Through the small 

 tube passes a rod carrying a glass thread at its extremity. This rod is 

 easily rotated by means of a milled head. The capillary glass thread is 

 attached to the extremity by means of beeswax. There is no revolving 

 collar, as it is quite unnecessary, especially when the Microscope is 

 provided with a revolving stage. By dispensing with the revolving 

 collar and making all movements depend entirely upon the adjust- 

 ments of the Microscope, greater stability and accuracy in working are 

 secured. 



* Amer. Journ. Micr., vi. (1881) pp. 149-51 (1 fig.)- 

 t See this Journal, ii. (1879) p. 952. 



