The President's Address. By E. M. Nelson. 



163 



seems to have been empirical, his one condition beiug secured by 

 placing the two lenses the difference of their foci apart. Boscovich 

 subsequently pointed out that the foci as selected by Huyghens fulfilled 

 another and important condition, viz. that half their sum was equal 

 to the distance of their separation, by which the achromatism of the 

 eye-piece was secured. It is hardly necessary to mention that achro- 

 matism in this eye-piece means something very different from achro- 

 matism in an objective. The achromatism in this eye-piece might be 

 appropriately termed " an achromatic effect," for it merely signifies 

 that the unequally magnified chromatic images formed by the field- 

 lens are unequally magnified by the eye-lens in such a manner that 

 they finally appear the same size. 



Theory. — It is to be feared that the action of the Huyghenian 

 eye-piece is but very imperfectly understood by microscopists, and no 

 wonder ; as an old and erroneous theory, published many years ago, is 

 copied in several text-books on the Microscope. The old theory was 

 that the objective image, which was slightly convex towards the eye, 

 was changed by the action of the field-lens into one that was concave 

 towards the eye-lens, and therefore in the best position with regard 

 to the eye-lens for obtaining a flat image. This, however, is incor- 

 rect : the objective image may be slightly convex towards the eye as 

 in the large dotted arrow in fig. 44, but the action of the field-lens 

 is to increase that convexity in the same direction, and most certainly 

 not to invert the curvature. 



With regard to distortion, which owes its existence to spherical 

 aberration causing too great a refraction of the excentrical pencils 

 which fall on the marginal zone of the field-lens, it will be better 

 perhaps to illustrate this defect in the eye-piece by means of a definite 

 example. Suppose we have a square grating focussed by the object- 

 glass, then the objective image will 

 be slightly barrel-distorted, but by 

 the influence of the field-lens it will 

 be reduced in size, and much more 

 barrel-distorted, as in fig. 41. Now 

 the action of a single lens in form- 

 ing a virtual image, as in a simple 

 non-achromatic Microscope, is to 

 enlarge an image and give it pin- 

 cushion-distortion, fig. 42 ; but as 

 the image at the diaphragm of the eye-piece, which the eye-lens 

 is magnifying, is already barrel-distorted, the eye-lens, by reason of 

 its pincushion-distorting power, neutralises this barrel-distortion, and 

 so makes the final image rectilinear. It would therefore be a mistake 

 to attempt to form a rectilinear image at the diaphragm of the eye- 

 piece, because the inevitable result of such a procedure would be a 

 final image that was pincushion-distorted by the action of the eye-lens. 

 It is therefore useless to aplanatise either lens without the other. 



Fig. 41. 



Fig. 42. 



