INTRODUCTION. 



an objective, to use with it cover- glasses of this definite 

 thickness. 1 But whoever does not fear the expense would 

 do well to provide himself at once with an objective for 

 " homogeneous immersion ". 2 They are all constructed 

 without correcting screw, since, as already indicated, the 

 thickness of the cover-glass, of course within the permissible 

 limits, is of no importance. By selecting a single such ob- 

 jective, say T V, and purchasing a series of eye-pieces, one can 

 obtain a range of possible magnification such as could only 

 be given by several water immersions, or dry objectives. A 

 system for homogeneous immersion, provided it is perfectly 

 constructed, can therefore replace several systems of another 

 kind. 



As a set-off against this, the use of a homogeneous immer- 

 sion, as indeed of a water immersion, requires special care, 

 especially in cleansing both it and the object. It must be 

 borne in mind that the immersion oil is a solvent of Canada 

 balsam, so that if this is used as a inountant it must be pro- 

 tected by a layer of gold size. While an oil immersion is, as 

 noted, largely independent of the thickness of the cover-glass, 

 should this happen to be very thin indeed, it is well to com- 

 pensate for it by slightly lengthening the draw-tube. 



Even in the smallest stands mentioned above, objectives 

 for homogeneous immersion can be used with great advan- 

 tage without any special apparatus for increasing the illumi- 

 nation ; but the highest capabilities of the homogeneous or 

 indeed any system are only brought out by the use of a sub- 

 stage achromatic condenser. Several of the stands referred 

 to above have substage condensers constructed specially for 

 them, and at a very moderate cost. 



The highest possibilities of a microscope are only attained 

 by the aid of the comparatively new " apochromatic " objec- 

 tives and their compensating eye-pieces, but their high price, 

 and the necessary accessories, may be taken for the present 

 to place them outside the use of those for whom this book is 

 intended. 



1 On this subject see a note on page xxx. jt 



2 Leitz, of Wetzlar, produces a T V of remarkable excellence, which has 

 stood the test of manufacture for nearly twenty years. Price 5. 



