88 NATURALISTS’ ASSISTANT. 
and the lens. It is thus possible to obtain a more brilliant 
view of the object as a larger amount of light can be passed 
through the objective. Besides this there are other immersion 
lenses, etc., in which oil, etc., take the place of the water. 
It may not come amiss to say that the objectives of dif- 
ferent makers, of the same nominal focal length, vary greatly 
in their magnifying power. This results from the fact that 
some manufacturers, in plain English, Ze about their lenses 
and sell for a fourth, for instance, a lens which in reality is a 
sixth or an eighth, and thus obtain a reputation for making 
lenses of wonderful power, while were their work tested upon 
its true merits its rank would be much less. One prominent 
American manufacturer notoriously does this and upon just 
this fraud has acquired a great reputation. 
Continental manufacturers have adopted an arbitrary sys- 
tem of numbering their objectives, and, for the convenience 
of many, the tables on the opposite page giving the equiva- 
lent of each in inches are inserted. 
Other prominent European, as well as the English and 
American makers designate their objectives by their focal 
length. The objectives of the Continental manufacturers are 
fully equal for work to those of English or American opticians 
while their prices are greatly lower, and the writer would 
here advise every one to buy the objectives of Hartnack -or 
Zeiss, ‘until American manufacturers offer their work at 
reasonable prices. 
The stage of the microscope should be firm and rigid. It 
is frequently convenient to have a stage of glass sliding upon 
brass supports, as thus a great smoothness of motion is ob- 
