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, lie 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- 



