PREFACE. 



to study what Professor Abbe has written, and then they 

 will probably come to the conclusion that the high-angle 

 school possess some forcible arguments, and may also 

 find that with wide apertures their battery of objectives 



may be considerably smaller to accomplish work of a better 







quality than when low angles are employed. 



Under the head of objectives, the almost total absence 

 of American productions will be remarked : it is only 

 recently that American objectives of the widest aperture 

 have found their way into the Author's hands. Their 

 definition is marvellous, but it has not been thought 

 advisable to include such objectives in a practical work 

 until after they have been used in ordinary observation for 

 a sufficient length of time. 



However, the reader may like to know that Spencer, of 

 Geneva, U.S.A., produces a 3-inch of I3,*a 2-inch of 20, 

 and a i-inch of 50; while Tolles, of Boston, U.S.A., makes 

 similar objectives, a ^ of 145; both of these opticians pro- 

 ducing the J-inch and all higher powers, of 180 air-angle. 



These wide apertures demonstrate clearly the accuracy 

 of the statement made on page 5 1, that wide angles require 

 more care in their correction, and are consequently more 

 expensive. Spencer's i-inch of 50 costs forty-five dollars, 

 or 9/. ; while the i-inch of 22 in their student's series costs 

 but ten dollars, or 2/. 



The whole of the information in the work has been 

 selected to aid the student as much as possible ; but it 

 should be remembered that the microscope is useful, from a 

 scientific point of view, only as an aid in research, unfolding 

 to us objects either invisible, or but faintly to be distinguished 



