INTRODUCTION. XXV 



inersion " objectives are now much in use. In these objec- 

 tives the cover-glass and the front lens of the object-glass 

 are connected by a drop of liquid. Such objectives are of 

 two kinds : " water," in which that is the liquid used, and 

 " homogeneous," in which the liquid is in general oil, or a 

 mixture of oils. The homogeneous immersion objectives are 

 dearer, less readily cleaned after use, and must be used only 

 with the special fluid for which they are manufactured ; but 

 on the other hand they transmit more light, bear a stronger 

 eye-piece, and, witbin certain wide limits, are independent 

 in their working of the thickness of the cover-glass. Dry 

 objectives of high power, and water immersions, are naturally 

 dependent on the thickness of the cover-glass which the 



FIG. 1. Objective with collar for correcting for thickness of cover-glass (Zeiss). 



light-rays pass through after leaving the object. To pro- 

 vide for this, they are usually manufactured also with 

 ' correction collar," for use according to the thickness of the 

 cover-glass, and at a somewhat- increased price. The correc- 

 tion collar accommodates the objective to the thickness of 

 the cover-glass which happens to be in use, by means of a 

 milled-headed screw which turns it, and adjusts the lenses 

 forming its optical system ; and upon it are divisions and 

 figures (see Fig. 1) which permit focussing for any given thick- 

 ness (within certain limits; of cover-glass, where this thickness 

 is known. A water immersion or a dry objective, without 

 correcting screw, is made to suit a certain medium thickness 

 of cover-glass, which is usually stated by the optician ; and 

 it is therefore preferable for the beginner, if he wants such 



