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 

 FJG , speak of these glasses. This expres- 



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 



