278 HOW TO SEE WITH THE MICROSCOPE. 



and that several trials have been ineffectually made ; all 

 of which lead to the same result. In this case the prob- 

 abilities are that the objective is at fault, and that any 

 attempt to work it by anyone, however expert, will 

 only result in a waste of time. The student ought not 

 to decide definitely until by practice he shall be entirely 

 competent to judge for himself. We mention the fact 

 here, because we know full well that some who may 

 read these pages will try and follow the author, employ- 

 ing such objectives as they may have at hand. Now it 

 might occur that an object glass capable of showing a. 

 medium Cherryfield or a Monmouth very well indeed, 

 even cutting the larger shells into checks or squares, 

 responding, too, to all the conditions we have presented, 

 and yet entirely fail when worked over any but an ex- 

 ceptionally large shell of the Leipsig. 



It may be apropos here to discuss another point which 

 is suggested as we write: It will be noticed that all 

 along in the course of these lessons we have made it oblig- 

 atory that the simplest illumination shall alone be em- 

 ployed, reservingi'or the present any allusion to special 

 methods used in the resolutions of the severest tests. 

 Our reasons are patent. First, the little lamp recom- 

 mended is in ordinary use, and is always at hand when 

 wanted; as to its size, the small lamp is even superior 

 to a larger one, while the smaller model has the advan- 

 tage of burning less oil, is handier to manipulate, and 

 is the more portable of the two. In a former part of 

 the book we have acknowledged the superior force due 

 to modified sunlight; nevertheless, the latter is unfit 



