ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



395 



records some of the results of his observations. Many of the pheno- 

 mena connected with the motion of diatoms, indicate that the frustules 

 are enveloped in a membrane which, if adhesive, would cause many of 

 the appearances noted, provided the motion be accounted for. Where 

 extraneous matter is seen trailing after a diatom it is, however, as 

 likely that the adhesive property resides in it as in the diatom. The 

 remarkable alternation of motion seems a very strong objection to the 

 ciliary theory and equally so to that of prehensile filaments. No other 

 ciliated or flagellate organism exhibits such alternations. Not even 

 in the case of large diatoms when moving with great force can any 

 trace of cilia or filaments be seen. If ciliary action or currents 

 produced by osmose were the true explanation, we should expect them 

 to move adjacent particles when the diatom is held fast, but yet free 

 particles are not moved nor is there any evidence of current in the 

 water, except where it is in contact with the diatom. In fact, none of 

 the suggested causes of motion explain satisfactorily all the phenomena 

 observed, and the problem still lies open to some persevering observer. 



MICROSCOPY, 

 a. Instruments, Accessories, &c. 



Griffith's Portable Microscope.*— Mr. E. H. Griffith has further 



modified this instrument, which now " has the usual coarse adjustment 



by rack and pinion, which is very accurately made, and by an ingenious 



addition, serves also as a fine adjustment. A ring is mounted on the 



axle of the hand-wheel ; a set-screw clamps the hand-wheel when the 



coarse adjustment is effected, so that 



it cannot be moved, and all danger P'ig. 61. 



of breaking the slide is avoided. Then 



a lever working in the ring moves the 



tubes by means of the same rack and 



pinion. As the lever is itself moved 



by a worm- screw, it forms a very exact 



and delicate focussing arrangement." 



Parkes' Class Microscope. — Messrs. 

 Parkes have adapted the Microscope 

 described ante, vol. i. (1881) p. 655, 

 for use as a Class or Demonstrating 

 Microscope. It is shown in Fig. 61. 

 The handle, in conjunction with the 

 base of the stand, enables it to be 

 placed on a table in the ordinary way 

 when so desired. The condensing lens 

 more usually employed when the instrument is being handed round 

 a class can be replaced by a mirror. 



Pringsheim's Photo-chemical Microscope. — Professor Prings- 

 heim's researches on the functions of chlorophyll in the life of the 



* Proc. Amer. Soc. Micr., 1881, p. 85. 



2 D 2 



