446 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



shows the application of these powers to all grades of work, from 

 that which is ordinarily done with a pocket lens to the extreme 

 limits of microscopical vision : — 



... It has not been my purpose to lay down any single set of 

 objectives as the only proper one, but to indicate the principles on 

 which selection should be made, and the relation of aperture to 

 amplifying power, and to show that there is at present no good theo- 

 retical reason for the use of objectives of greater amplifying power 

 than the 1/8 in." 



Dr. Blackham, it will be seen, advocates the use of eye-pieces as 

 high as 1/4 in. which is largely in excess of Prof. Abbe's figures, 

 which do not go beyond an amplification of 15 times.* 



Mr. J. D. Cox believes f " Dr- Blackham has the verdict of ex- 

 perience with him when he says four or five lenses with a proper 

 number of eye-pieces will cover the whole range of microscopical 

 examination. In such a number of lenses you may get all the 

 necessary combination of the three qualities of angle, power, and 

 working distance which you may need. Different investigators may 

 choose different series, but no one need have a greater number in the 

 series. Economy is to be considered in deciding whether we shall 

 choose one or another lens ; but this is also consistent with the state- 



* See this Journal, iii. (1883) p. 808. 



t Proc. Amer. Soc. Micr., 6th Ann. Meeting, 1883, pp. 229-30. 



