174 SELECTION AND USE 



it will be found that each objective has its own special charac- 

 teristics, which must be carefully studied by the owner if he 

 would command success. 



The difficulty of giving any rules which will enable the 

 student to put the "finer touches" on this kind of work, is well 

 set forth by Dr. Blackham in a recent article,* from which we 

 quote as follows : 



"It will probably be expected that something should be said 

 here in reference to the adjustment of the objective for dif- 

 ferent cover-glasses, etc., by means of .the screw collar, but on 

 this point, unfortunately, but little can be said, though, of 

 course, it is a most important one, and- the better the objective, 

 and the wider its angle, the more important is accurate adjust- 

 ment. Every wide-angled immersion objective that is worth 

 having, is a separate work of art, and, as such, has an individ- 

 uality with which the worker must become acquainted, and 

 which he must learn to turn to his advantage. 



" None of these lenses which I have seen are perfectly achro- 

 matic, and each has a special wave length at which it does its 

 best. In Tolles', and Bausch & Lomb's, and I believe in 

 Spencer's lenses, this is between the blue and the green, but 

 the exact shade differs with different lenses, and must be found 

 by experiment. My plan is to adjust roughly by means of the 

 tint of the field, then to bring an object into the field (if we 

 are at work on the Probe Platte, one of the easier diatoms, say 

 Pleurosigma angulalum], and focus on it and arrange the illu- 

 mination as accurately as possible, and then with the finger and 

 thumb of the left hand, turn the correction collar of the objec- 

 tive backwards and forwards, keeping the object in focus all 

 the time by means of the forefinger of the right hand on the 

 milled head of the fine adjustment, until the best effect is ob- 

 tained. An occasional slight change in the position of the mir- 

 ror is often needed. 



"In all these manipulations, deliberation and care are 

 needed, and the patience of the beginner will often be sorely 

 taxed, but let him remember that nothing worth having, can be 

 gotten without trouble." 



* American Journal of Microscopy for February, 1880. 



