176 PBACTICAL TREATISE ON THE MICKOSCOPE. 



fourth, and, in some cases, the one-half; in all the powers lower 

 than these, the adjustment is not required, as the thickness of 

 the glass cover does not materially interfere with their definition. 

 The eye-pieces supplied with the compound microscopes 

 are of the Huyghenian principle before described, and are 

 generally three in number; they may be designated by the 

 letters. A, B, and C, the first being the lowest in power, the 

 last the highest. Those of Messrs. Powell and Smith are so 

 constructed, that with the one-inch object-glasses the magni- 

 fying power with A will be about 30, with B 60, and C 100, 

 or in the proportion of 1, 2, 3 ; but Mr. Ross varies the power 

 of the second or B, and his proportion is about 1,1^ and 2 4. 

 The lowest, or that marked A, is the one more commonly 

 employed, the others, especially C, are not so frequently used, 

 in consequence of the loss of light, and, in some cases, of defi- 

 nition also ; but all are extremely necessary when we wish 

 to gain an increase in the magnifying power without the 

 employment of a higher object-glass. Two other forms of 

 eye-piece are sometimes employed, one of which was invented 

 by Ramsden ; it consists of two planoconvex lenses with their 

 convex sides towards each other, and is used with the micro- 

 meter. If a divided glass scale or fine wires be placed exactly 

 where the image formed by the object-glass is situated, the 

 scale and the image will be magnified together, and every 

 part of the image will be, as it were, brought in contact with 

 a scale, the value of each of the divisions of which can be accu- 

 rately ascertained, even to the twenty-thousandth of an inch. 

 The curvature of the image formed by this eye-piece is not 

 reversed as in the Huyghenian form, hence it has received the 

 name of a positive eye-piece, in contradistinction to that of 

 Huyghens, which is termed a negative one; but however 

 accurately such an eye-piece may be made, its defining powc r 

 is not so good as that of Huyghens, but Mr. Ross states, that if 

 the glasses were made up of two achromatic combinations, this 

 eye-piece would be by far the most perfect of all, both for 

 telescopes and microscopes. Some persons employ either two 

 plano-convex or double convex lenses as a substitute for the 

 single eye-lens of the eye-piece, in order that the field of view 



