260 THE SIMPLE MICROSCOPE. 



3. CURVATURE OF THE FIELD OF VIEW. 



The so-called " curvature " of the field of view cannot be ex- 

 plained by the arching of the image-surface, but is due to the 

 deviation of the pencils reaching the eye that is, of their axes. 

 On this point our previous discussion of the flatness of the field 

 of view, and the action of the eye-lens, will hold good in general. 

 The special application of those considerations to the cylindrical 

 niagnifying-glass, the doublet, &c., is facilitated inasmuch as the 

 path of the incident and emergent pencils is almost exactly the 

 same as in the Kamsden eye-piece. The point of convergence of 

 the emergent pencils is determined by the position of the observ- 

 ing eye ; it stands in the same relation to the point of intersection 

 of the incident rays, produced backwards, as an object to its image, 

 and may, therefore, always be chosen so that the latter, as in the 

 eye-piece, is at a distance of about 200 mm. from the refracting 

 surfaces. The coincidence is therefore complete. 



For the elimination of the distortion of the image the refracting 

 surfaces must be so combined that they form a system as far as 

 possible aplanatic for the given focal lengths i.e., for the distances 

 of the points of intersection. The arching of the image-surface is 

 not dependent upon this ; it is due to the fact that the object-points 

 are not at equal distances from the refracting surface-elements. 



4. THE MAGNIFYING POWER. 



The magnifying power is given by the well-known formula 



/ p* f -j- p* 



m = - j^> r since p* is here negative, m = - . Since^* 



J J 



must be reckoned from the posterior principal plane, it is evident 

 that with a given distance of vision the amplification is the lower 

 the further the eye is from this principal plane, or indeed from the 

 system of lenses. It is therefore the general rule to place the eye 

 as near as possible to the last refracting surface. 



