MAGNIFYING POWER. 77 



largest angles, the visual effect is sensibly impaired when 

 the rays are transmitted through any other thickness of 

 covering-glass than that for which they have been specially 

 corrected." 



The value of penetrating power is especially felt when 

 the binocular arrangement is employed, since the assistance 

 which it is able to give in the estimation of the solid 

 forms of objects, is altogether neutralized by the employ- 

 ment of objectives of such wide angular aperture as not to 

 show any part of the object, save what is precisely in 

 focus; and, therefore, there cannot be a doubt of the supe- 

 rior value of objectives of a moderate angle of aperture 

 for ordinary working purposes. 



" Successful observation, with very high power, is 

 mainly dependent upon illumination. Indeed, by ordinary 

 means, it is not possible to obtain a light sufficiently in- 

 tense to illustrate an object magnified 3,000 diameters. 

 I have tried various plans, and have come to the conclu- 

 sion, that the most satisfactory results are obtained by the 

 use of Kelner's eye-piece as a condenser. By this means I 

 can obtain a light sufficient for a magnifying power of 

 10,000 linear. 



" My method of increasing the size of the image, with- 

 out altering the object-glass, is as follows : 



" Supposing the limits of magnifying power of the 

 object-glass to have been reached, I then increase the 

 distance between it and the eye-piece, by lengthening the 

 body by the aid of a draw-tube. The 2*5 th objective 

 being applied when the tube is increased in length, so 

 that from the lowest glass of the object-glass to the eye- 

 glass of eye-piece, the distance measures 24 inches, the 

 magnifying power corresponds to upwards of 10,000 dia- 

 meters, 20 inches about 6,000, 15 inches about 2,600, 11 

 inches about 1,800. 



" If there is any reflection from the interior of the tube 

 which renders the image indistinct, let the tube be lined 

 with black velvet ; of course the practical utility of in- 

 creasing the magnifying power entirely depends upon the 

 character of the specimen, and the preparations best 

 suited for examination by very high powers must be im- 

 mersed in the strongest glycerine that can be procured 



