50 



CONSTRUCTION OF THE MICROSCOPE. 



is true of the intermediate colours and of the other 

 pencils. The eye-glass e e not only brings together the 

 images 6' V, r r, but it likewise has the most important 

 effect of rendering them flat, thus at once correcting both 

 the chromatic and spherical aberration. 



The Huyghenian eye-piece, which we have described, is 

 the best for merely optical purposes ; but when it is 

 required to measure the magnified image, 

 we use the eye-piece invented by Mr. 

 Ramsden, and called by him the micrometer 

 eye-piece. The arrangement may be readily 

 understood upon reference to fig. 33. The 

 field-glass has now its plane face turned 

 towards the object ; the rays from the ob- 

 ject are made to converge immediately in 

 front of the field-glass ; and here is placed a plane-glass, 

 on which are engraved divisions of 1 -100th of an inch or 



less. The markings of these 

 divisions come into focus, 

 therefore, at the same time 

 as the image of the object, 

 and both are distinctly seen 

 together. The glass with its 

 divisions is shown in fig. 34, 

 on which, at a, are seen some 

 magnified grains of starch. 

 Thus the measure of the 

 magnified image is given by 

 mere inspection ; and the 

 value of such measurements, 

 in reference to the real object, when once obtained, is con- 

 stant for the same object-glass. 



It is afiirmed by Mr. Ross, that if the achromatic prin- 

 ciple were applied to the construction of eye-pieces, the 

 latter is the form with which the greatest perfection would 

 be obtained. That such an adaptation might be produc- 

 tive of valuable results, appears from Mr. Brooke's state- 

 ment, that ho has employed as an eye-piece, a triplet 

 objective of one inch focus, the definition obtained by it 

 being superior to that afforded by the ordinary Huyghe- 

 niaa eye-piece. Some of the lowest French achromatic 



Fiff. 34. 



