48 PRACTICAL MICROSCOPY. 



extremely difficult to procure an equal illumination of the 

 whole field in fact, the general performance of the instru- 

 ment is seriously interfered with, and therefore, for use 

 with the binocular, short mounts should be preferred and 

 the longer ones rejected. 



Mr. Swift has recently issued a series of objectives in 

 short mounts, especially for use with the Wenham binocular, 

 each of which allows of perfect illumination, without the 

 aid of an achromatic condenser. They range from i inch 

 upwards. 



Objectives are generally spoken of in terms of the ampli- 

 fication which they yield, the standard of comparison being 

 the magnification given by a single lens of the nominal focus. 

 The student must not, therefore, imagine that an objective 

 stated to be i-inch, -inch, or so on, will focus at these 

 distances from the object. Opticians have never used the 

 term in that sense, though a few writers in public journals 

 seem to have understood the nomenclature in that light. 



The degree of amplification, taken with the aperture, is 

 the surest guide to the focal distance ; for as the aperture 

 increases, the less will the working distance be, a point to 

 be considered in the selection of an objective for students' 

 use. 



The lower powers are often made to separate. An 

 objective giving the amplification of a 2-inch when com- 

 plete, is converted into a 4-inch by removing the anterior 

 combination. Separating 2-inch and i-inch, i-inch and 

 J-inch, J-inch and i-inch, are also made, with others ; but the 

 student is advised not to invest much in these separating 

 powers. 



The best low-power objectives for general work will be 

 found to possess the following air angles : 9 for the 

 4-inch, 15 for the 2-inch, while the i-inch seems to per- 

 form most satisfactorily at 30. 



