12 



found by multiplying its number by the numerical value of the initial magnification of the 

 objective. For instance, an objective of 3 mm focus combined with eye-piece 12 gives a 

 magnification of 12 X 83.3 = 1000 diameters. 



The exceptionally low power eye-piece 2 , known as the Searcher eye- 

 piece, serves to extend the effectiveness of the various magnifications required 

 to be successively used with any one . objective as far as possible downwards 

 through the lower strata of a specimen also, so as to facilitate preliminary 

 examination and the process of searching for particular points of the object 

 when high-power objectives are being used. By obviating in many instances 

 the necessity of a change, this searcher is particularly useful in combination with 

 immersion objectives, where great inconvenience is caused by having to change 

 a lens already adjusted for another of longer focus. 



The Working' eye-pieces vary in magnifying power from 4 upwards, and 

 even their highest powers can be used with the greatest comfort. In all of them 

 the eye-point (the pupil of emerging rays) lies sufficiently high above the 

 upper lens surface, and the diameter of the lenses is sufficiently large, to com- 

 pletely obviate the inconveniences formerly attending the use of eye-pieces of 

 short focus. 



We have recently modified one of the eye-pieces of the 

 compensating series (as well as one each of the series of pro- 

 jection and of the HUYGENTAN eye-pieces). It is a modified 

 eye-piece 4 with a collective lens of enlarged diameter and is 

 distinguished by the designation Compensating Eye-piece 4*. 

 Its visual field as compared to that of the ordinary com- 

 pensating eye-piece 4 is enlarged to the proportion of 3:2. 

 To use this eye-piece (Fig. 4) the draw-tube with the sliding 

 sleeve is removed from the body of the microscope and eye- 

 piece 4* then screwed on in its place. It is fitted with a 

 revolving collar and can be fixed rigidly in any desired position 

 by means of a clamping screw K. An iris -diaphragm, by 

 means of which the area of the field may be varied at will - 

 frequently a great convenience in demonstrations takes the place 

 of the ordinary fixed diaphragm. The eye-lens is mounted in 

 a sliding sleeve, after the manner of our micrometer eye-pieces, 

 thereby adapting it for use with a scale, &c., when desired. 



Fig. 4. 



Compensating 1 Eye-piece J-* 

 with iris-diaphragm. 



(Vs Full Size.) 



The enlargement of the area of the visual field is, however, of real practical value 

 only in the case of the lower-power objectives of 16mm and 8mm; because the sharp field 

 of the higher powers is not sufficiently large to satisfactorily fill the field of this eye-piece. 



Regarding the use of this eye-piece with a special form of Professor ABBE'S drawing 

 apparatus see page 89. 



