INVEKTEBRATA, CRYPTOGAMIA, MICROSCOPY, ETC. 



513 



ing aplanatic focus ; and the value of a in both the sets is taken from 



the formula 



A . M 

 a — —===: » 



V 1 - u^ 



the increments of u being equal ; e. g. the values of m = .... • 2 

 ... .0*4. ...0-6. ...0-8. Provided the common centre of all 

 tlie "ciirves in the diagram Fig. 36 is adjusted to lie on the axis, and 

 its plane is perpendicular to the axis and at the proper distance A 



Fig. 3G. 



(A = 12 mm.) 



from the aplanatic focus, the diagram appears in the image as two sets 

 of equidistant parallel lines intersecting at right angles.* The parts of 

 the diagram bounded by curved lines and increasing continually 

 towards the extremities in size and distortion appear as equal 

 squares ; the iutcrsectious of the hyperbolas (forming angles which 

 towards the outside become increasingly acute and obtuse) are repro- 



* Image of the hyperbolic curves, as depicted by a wide-angled aplanatic 

 system, under the conditions named above — the dotted circle indicates the limit 

 of the telescopic field corresponding to an air-angle of 180°. 



Fig. 37. 



/^ jiL ^ _• >_ ^ ^ 



/-- — ^ ^ ^ — — 



