28 MARVELS OF POND-LIFE. 



tlie instrument, that is to say on one side of the 

 space the body would occupy if it were prolonged. 

 By this means, and by placing the lamp at an angle 

 with the mirror, that must be learnt by experiment, 

 all the light that reaches the eye has first passed 

 through the object, and is refracted by it out of 

 the line it was taking, which would have carried 

 it entirely away. Or the object may be illuminated 

 by an apparatus called a spotted- lens^ which is a 

 small bull's-eye placed under the stage, and having 

 all the centre of its i'ace covered with a plaister 

 of black silk. In this method the central or 

 direct rays from the mirror are obstructed, but 

 those which strike the edge of the bull's-eye are 

 bent towards the object, which they penetrate and 

 illuminate if it is sufficiently transparent and re- 

 fractive. Another mode of dark ground illumination 

 is by employing an expensive instrument called a 

 ])araboIic illuminator^ Avhich need not be described. 



Different specimens and species of Vorticellw vary 

 in the length of their bells from one three or four- 

 thousandth to one-hundred and twentieth of an inch, 

 and when they are tolerably large, the dark ground 

 illumination produces a beautiful effect. The bells 

 shine with a pearly iridescent lustre, and their cilia 

 Hash with brilliant prismatic colours. 



The VorticelJiiia belong to the upper division of 

 the Protozoa — i\\Q cilicfta^ or ciliated animalcules, and 



