30 The Eye and the Microscope. 



I)e no glare. With, a naked lamp this can be avoided by 

 regulating the quantity of the lights and paying attention to 

 the angle — a little obliquity having a softening effect. A 

 ground glass globe gives a pleasant repose, and the same, or a 

 better effect may be obtained by melting a bit of spermaceti 

 on thin paper, and cutting a disk of the preparation thus made, 

 which may, according to its size, be dropt over the largest hole 

 of the diaphragm, or placed on the flat side of the bull's-eye 

 lens. The latter is the best plan, as it causes no interference 

 with an oblique direction given to the light by turning the 

 mirror. The student must not imagine he has illuminated the 

 echinus' spine properly until every portion is clear; no part 

 Ibeing enough in the shade to diminish its distinctness, and 

 none too bright to be seen without pain. 



Having thoroughly succeeded in. illuminating the echinus' 

 spine as a transparent object on a light ground, so that it can 

 he seen as comfortably as a willow-pattern plate one foot from 

 the eye, let the mirror be thrown out of the plane of the instru- 

 ment, and at such an angle that none of its light can fall 

 directly upon the object-glass. If this be accomplished, the 

 ground, will be dark, and the object will stand out in strong 

 relief seen by the light it is able to refract, or bend back to the 

 plane of the instrument. This produces a beautiful effect, and 

 is sometimes superior to the dark ground illumination afforded 

 by the parabola or spotted lens. What has been previously said 

 about uniform distribution of light should still be attended to, 

 as the want of this uniformity is a frequent cause of distress to 

 the eye. 



It is customary to treat the parabolic illuminator as if it 

 were a difficult instrument to mac age, but after a month or 

 two's practice with the stage mirror at various angles, it may 

 be advantageously employed, and has an admirable effect, not 

 only in giving beauty to a large range of objects which possess 

 the necessary refractive power, but in rendering visible points 

 of structure which other modes of illumination do not readily 

 disclose. For example, the red eyes of rotifers gleam like 

 xubies in its light. The action of the parabolic illuminator is 

 to throw through an object a cone of rays at such an angle that 

 none shall enter the objective until refracted by the substance 

 under view, which, if suited to the purpose, stands out with 

 great brilliance on a black ground. The stop with which it 

 is provided should usually be pulled back and a strong light 

 thrown up the instrument from the stage mirror, which should 

 receive a good supply of light from the bull's-eye or condens- 

 ing lens, having in this case its fiat side to the lamp. It is 

 usual to recommend the use of the plane mirror, but the con- 

 ■cave one generally answers best. The parabola should be 



