72 PRACTICAL TREATISE ON THE MICROSCOPE. 



would be required to be of the same focal distance as that 

 given to the others by name. In other words, if a single 

 lens were made the object-glass of a compound microscope, 

 and if it were necessary to employ a power equal to that of 

 the one-fourth achromatic combination, with the same com 

 pound body, it would be found that a thin single lens of one- 

 quarter of an inch focus would be required to give that 

 power. It would be more useful in practice if the name 

 given to each of the object-glasses were expressive of the 

 magnifying power instead of being derived in the manner above 

 described; if we take, for instance, the glasses called the 

 half-inch, as constructed by each of our three eminent makers, 

 and compare them together, we shall find that all three wiU 

 differ, more or less, in their magnifying power, but stiU they 

 all bear the name of half-inch ; neither do two glasses, similar 

 in name, even of the same maker, always agree exactly ; hence 

 it would be very desirable in practice to apply a term to them 

 which should express their magnifying power, but such a 

 nomenclature could not at this advanced period be easily 

 carried out. 



The eye-pieces supplied with the compound achromatic 

 microscopes are generally three in number, and the form 

 employed is that known by the name of the Huyghenian, it 

 having been first employed by Huyghens for his telescopes. 

 Each one consists of two planoconvex lenses placed at a dis- 

 tance from each other equal to half the sum of their focal 

 lengths ; the plane surfaces of the lenses are towards the eye, 

 and that nearest the eye is termed the eye-glass, whilst that 

 most distant is termed the field-glass. A stop, or diaphragm, 

 is placed about half-way between the two lenses, this arrange- 

 ment was adopted by Huyghens for the purpose of diminishing 

 the spherical aberration, by producing the refraction at two 

 glasses instead of one, and of materially increasing the field of 

 view; but it was reserved for Boscovich to point out that 

 another valuable property of this eye-piece was the correction 

 of a great part of the chromatic aberration as well. This 

 subject has been since critically examined by Mr. Varley, 

 and to his paper, in the fifty-first volume of the Transactions 



