98 NATURALISTS’ ASSISTANT. 
sion objective. Most objectives use water for an immersing 
medium but some are adapted for oil of cedar, glycerine or 
other liquid. Immersion objectives are valuable for some 
special purposes, but for ordinary work the “dry” lenses are 
much better. 
Most high powers of American objectives have an adjust- 
ment for thickness of cover glass. This is only necessary for 
objectives of very high angle (and the higher the angle, be- 
yond a certain point, the more use- 
less the objective). It is better as 
well as much cheaper to purchase 
objectives without this adjustment 
and then use the thinnest cover 
glasses made. These objectives 
without adjustment are always well 
corrected and give good results. 
Having mentioned high angle 
lenses above, it may not be amiss to 
speak of these glasses. This expres- 
Fic. 34. 
sion refers to the angular aperture of 
the objective, or, in other words, the angle which the ex- 
treme rays of light make with each other in entering the 
objective, and the larger the angle the “higher” it is. 
With every increase in the angular aperture, a shorter 
working distance of the lens is necessary, and so with very 
high-angled lenses the inconvenience of working far more 
than counterbalances the gain in definition. Some makers 
absurdly claim to make objectives of 180° angular aperture ! 
People making such claims should be carefully avoided, as 
