198 MICROSCOPIC ILLUSTRATIONS. 



engiscope was: I can give no better directions concerning" 

 it. The sapphire and diamond lenses have their own 

 peculiar settings in a species of dish or eup, which is 

 attached to the arm by being thrust into the hole at its 

 extremity. The superior cohesion of the precious stones 

 admits of their being burnished into a cell, which is again 

 fixed into the setting. This arrangement allows the 

 lenses to be shifted atout without difficulty, or risk of 

 losing them. If they are J)lano-convex, as they should 

 be, another advantage results, which is, that they may 

 be used either'with their flat or convex side towards the 

 object, according as great distinctness, or a large field of 

 view, may be required (for you cannot have these quali- 

 ties combined). Remember, therefore, if you want a 

 large field of view, and do not, at the same time, require a 

 large aperture, united with great distinctness, to place the 

 lens with its flat side next the eye. If, on the contrary, you 

 must have a very large aperture, conjoined with the utmost 

 possible degree of distinctness, then place the convex side 

 next your eye. The aplanatic lenses should also have set- 

 tings on which they can be screwed at pleasure, to be used 

 as simple microscopes. If, at any time, when bodies are 

 merely laid upon the stage, and confined to it only by their 

 gravity (the stand being used vertically, as in dissections, 

 for example), the face should come into too close contact 

 with the stage, the apparatus (fig. 21) must be used, 

 which is inserted into the aperture, like the live-boxes and 

 slider-holder : the object being laid upon it, due space will 

 be aiForded for the nose, and the breath will be less likely 



