ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



681 



the last we have the full aperture. Any intermediate degree of 

 illumination can be obtained ; the illumination is made excentric by 

 shifting the whole apparatus laterally. 



An analogous device was constructed by Dollond, and is described 

 and figured by Harting from a Microscope at Utrecht.* A practi- 

 cally identical form which we recently obtained in England, is shown 

 in fig. 134, where two plates with V-shaped apertures are made to 



Fig. 132. 



Fig. 133. 



Fig. 134. 



move simultaneously in opposite directions by racks and a pinion. 

 The aperture can thus be varied from a pin-hole to half an inch. 

 Deutgen's Micrometer-Microscope (supra, p. 673) has the same form 

 of diaphragm, which is however a fixture beneath the stage. 



Now that the Iris diaphragm, however, in the form used by 

 Messrs. Beck in their " Star " Microscope, can be made so cheaply, it 

 would appear to supersede any of the forms of diaphragm above 

 noted. 



Lieberkiihn Stops. f — Dr. G. W. M. Giles writes that during the 

 process of examination and delineation it will be often found desirable 

 to substitute direct for transmitted illumination, and to effect this 

 change expeditiously he finds no appliance so useful as the old- 

 fashioned but much-neglected Licberkuhn. To stop out the central 

 rays of light he employs small discs of vulcanite, sawn out of a very 

 thin piece of sheeting. By simply wetting them, these can be made to 

 adhere to any part of the under surface of the slide, and can be shifted 

 about if necessary with the tip of the finger, without removing the 

 slide from the stage. By alternately employing direct and trans- 

 mitted light, many details of structure can be learnt which could not 

 possibly be made out by either alone, and it enables one also to fill in 

 the natural colours in the finished drawing, which are quite lost by 

 transmitted light. 



Ross's Centering Glass. — This apparatus was designed by Mr. 

 A. Boss for ascertaining whether stage diaphragms, illuminators, and 

 other appliances are properly adjusted in the optic axis of the Micro- 

 scope, and acts on the principle that when suitable lenses are inserted 

 in the body, or superadded to the eye-piece at various positions, they 



* Das Mikroskop, 1859, pp. 841-2 (2 figs.)- t Sci.-Gossip, 1886, p. 121. 



