148 HOW TO SEE WITH THE MICROSCOPE. 



ADJUSTABLE OBJECTIVES. 



The generally received notion in regard to these 

 glasses is, that they are provided with an adjusting 

 collar, so that they may be " corrected for aberrations," 

 due to the thickness of cover employed on the mount 

 under observation. This is all true in a certain sense, 

 and tc a certain extent only. If we examine the same 

 object under the same objective (a first-class glass of 

 high aperture), but through covers of varying thickness, 

 we will find that the very best performance of the glass 

 will be obtained when worked over a particular thick- 

 ness of cover, and that any change from this particular 

 thickness will interfere with the performance of the 

 lens, no matter how perfect the corrections in each case 

 may be made. My own experience teaches me that the 

 maximum definition is only obtained when the objec- 

 tive adjusts at or near that point in the collar adjust- 

 ment which corresponds to the maximum aperture of 

 the objective. Now, in most glasses of high angles the 

 maximum angle will be found at only one position of 

 the collar. I say " most objectives," for we have a 

 one-tenth made by Mr. Tolles, that has maximum angle 

 nearly at any point within the range of its collar ad- 

 justment. We have also a one-sixth by the same 

 maker, which has maximum angle only at the " closed 

 point," the aperture decreasing rapidly as the collar 

 approaches " open," and with the decrease of angle there 

 is an accompanying decrease of definition and effective 

 force of the objective. 



