254 SUPPLEMENT TO THE 



to the object an arrangement sometimes, though rarely 

 necessary, with certain objects not of the lined kind. The 

 objective end of the engiscope may then be introduced 

 into the interior of the slider-holder. It is impossible to 

 lay down rules concerning the angle of deviation from 

 direct light, which it is most advisable to employ, in order 

 to bring out any particular set of lines most satisfactorily ; 

 this must be left to the taste of the observer : neither can 

 the distance of the candle from the object (on which the 

 intensity of the illumination depends) be determined, so as 

 to meet the views of every microscopist ; from two to six 

 inches may, however, be roughly stated. Whatever dis- 

 tance, and whatever degree of obliquity, brings out the lines 

 darkest, and the spaces between them clearest, according to 

 the vision of the individual who uses the instrument, is the 

 best* 



The following rule will serve to shew when the light is 

 direct, and when it is oblique. If the penumbra of the 

 object, when put out of focus, appears stationary, and 

 to go within and without the focus without changing its 

 position, the light is direct, supposing the object-glass 

 to be in adjustment ; but if it seems to travel from one 

 margin of the field of view to the other, then the light is 

 oblique, and the direction of it is in the line in which the 

 object seems to travel. 



* In order to procure specimens of lined objects in the best possible state 

 for observation, it is recommended to naturalists to steep the insects from 

 which they are derived in sulphuric aether ; this dissolves the grease which 

 frequently tarnishes and clogs up their minute lines and markings, while it 

 accumulates and licks up dust, &c., thus rendering them unfit for observation ; 

 on removing the insect from the fluid, the aether will evaporate rapidly, and 

 leave it dry, when the scales may be removed. Delicate scales can only be 

 procured perfect from living specimens. 



