ZOOLOGY AND BOTANY, MICKOSCOPY, ETC. 135 



Not being clear as to the mode in which the designer intended his 

 apparatus to be used, we applied to him on the subject, but without 

 receiving any reply. Mr. G. C. Karop has, however, kindly furnished 

 us with the following note : — 



" I do not think the use of this compressor necessarily refers to a 

 temporary closure only. You have a specimen which cannot be 

 satisfactorily examined except under pressure ; the effect of pressure 

 you may wish to keep and exhibit. A specimen is placed on a slip 

 with, say, a drop of glycerin or other preservative ; a cover-glass is 

 placed on this, and the whole is transferred to the compressor-plate, 

 the three curved springs being in position on the cover. The whole 

 is then put on the stage of the Microscope, and the construction allows 

 of the objective working down through the ring, whilst sufl&cient 

 pressure is obtained by the micrometer screw to show the desired 

 points. This being done, the apparatus is removed from the stage, 

 any surplus glycerin, &c., wiped oflF, and the preparation sealed by 

 paraffin with a hot wire, according to the well-known method. When 

 dry it is put on a turntable and permanently sealed by a ring of 

 Paris glue or white cement, &c." 



Martius' Method of Determining the Absolute Rate of Ciliary 

 Vibration by the Stroboscope.* — By the stroboscope, as is well 

 known, a vibrating body is instantaneously illuminated or is viewed 

 at successive intervals through a revolving or vibrating aperture. 



A familiar instance of this is the " wheel of life " toy sold in the 

 streets a few years ago. The wheels of a carriage, or a moving animal, 

 seen by the light of a flash of lightning, appear perfectly stationary, the 

 duration of the light being so brief as to admit of only an inappreciable 

 movement of the body while illumination lasts. If a regular succes- 

 sion of light flashes is produced, the moving body will be seen in as 

 many diff'erent positions as there are flashes of light. If a body 

 rotating rapidly on a fixed axis be viewed by light flashes occurring 

 once during each revolution of the body, only one image will be 

 observed, and this will result from a succession of impressions upon 

 the retina, which by the persistence of vision become blended into 

 one continuous image. In this case no movement of the body will 

 be apparent ; but if the flashes of light succeed each other ever so 

 little slower than the rotatory period of the revolving body, the body 

 will appear to move slowly forward, while in reality it is moving 

 rapidly ; and should the light flashes succeed each other more rapidly 

 than the revolutions of the revolving body, the body will appear to 

 move slowly backward, or in a direction opposite to that in which it is 

 really turning. These curious effects are also produced when the 

 number of the light flashes is a multiple of the number of revolutions, 

 or vice versd.^ 



* Arch. f. Anat. u. Physiol. (Physiol. Abtheil.) 1884, pp. 456-60. 



t The preceding paragraph is interpolated from an article by Mr. G. M. 

 Hopkins (' Scientific American ') in which he describes the method he used for 

 applying intermittent light to a microscopical examination of ciliated organisms 

 by an electrically rotated aperture disc, arranged to interrupt the beam of ligW 

 employed in illuminating the object to be examined. 



The instrument consists of a single electric motor mounted on a plate having 



