' 186 THE MICROSCOPICAL NEWS. 
have for the ordinary cements and varnishes, such care is necessary 
in the use of objectives constructed to work with them, as can only 
be given when the observer desires to make a very minute and 
critical examination of a securely mounted object.” 
A table is then given which in addition to the magnifying-powers 
of objectives with the A and B eye-pieces also “specifies the angle 
of aperture which, in the writer’s judgment, is most suitable for 
each. He has the satisfaction of finding that his opinions on this 
latter point, which are based on long experience in the microscopic 
study of a wider range of animal and vegetable objects than has 
fallen within the purview of most of his contemporaries, are in 
accordance with the conclusions drawn by Professor Abbe from 
his profound investigations into the theory of microscopic vision, 
which have been carried into practical accomplishment in the 
excellent productions of Mr. Zeiss.” An extract from the table will 
be found on the next page. 
“For ordinary biological work, the 1/8, 1/10, and 1/12 objectives, 
with angles of from 100° to 200’, will be found to answer extremely 
well if constructed on the water-immersion system.” 
| Focal Angular || Focal Angular 
Length. Aperture. Length. Aperture. 
in. (a) in. o 
4 9 1/4 50-80 
3 I2 1/5 95 
2 15 | 1/6 110 
1} 20 | 1/8 140 
I 30 1/10 150 
2/3 40 | 1/12 160 
1/2 45 | T/T6) 77 170 
4/10 70 
“Tt must be understood that there is no intention in these re- 
marks to undervalue the efforts which have been perseveringly 
made by the ablest constructors of microscopic objectives in the 
direction of enlargement of aperture. For these efforts, besides 
increasing the resolving-power of the instrument, have done the 
great service of producing a vast improvement in the quality of 
those objectives of moderate aperture which are most valuable to 
the scientific biologist ; and the microscopist who wishes his arma- 
mentum to be complete will provide himself with objectives of those 
different qualities as well as different powers which shall best suit 
his particular requirements.”—/. &. JZ. S. 
