414 Dr. Goring on Achromatic Microscopes, 



qualities which are incompatible with them, let it be known 

 that they have only to apply the ordinary quadruple and 

 quintuple eye-pieces to the achromatic object-glass, and 

 they will obtain the number of degrees of extension which 

 they want, (for the size of a field of view, as well as the 

 flatness of it, depends almost entirely on the nature of the 

 eye-piece employed,) at the moderate expense of ruining the 

 brilliancy and sharpness of the image, and introducing such 

 a flare and reflection from the numerous surfaces, as will, 

 under certain circumstances, give an image or two more than 

 are wanted, with a bright luminous spot in the centre of the 

 field! 



There is a defect in the double achromatic eye-pieces for 

 microscopes which cannot well be helped : it is, that when 

 glazed in such a manner as to be truly achromatic (in which 

 case the field-glass will be of longer focus and farther removed 

 from the eye-glass than in the telescopic ones), a certain dis- 

 tortion, or convex, barrel-shaped curvature is seen at the 

 edges of the field of view : if, again, this defect is remedied by 

 a different mode of glazing, they will be no longer achroma- 

 tic ; so there is only a choice of evils. 



Though a common erecting eye-piece does well enough for 

 the small aplanatics when its third and fourth glasses are 

 placed at the same distance from the object-glass as the other 

 inverting eye-pieces are ; yet, if it is thrust in nearer to the 

 object end to lower the power (which it is frequently con- 

 venient to do), a great deal of colour becomes visible, at least 

 if the stop between the erecting-glasses is sufficiently enlarged 

 to allow the whole pencil to pass through : — under these 

 circumstances the necessity of introducing a secondary achro- 

 matic becomes evident ; but, in order to make a workmanlike 

 job of the thing, it would seem to require to be made in two 

 .pieces, viz. the fourth a compound of a convex and concave, 

 but so much over-corrected as to neutralize the aberration of 

 another convex at some distance, standing in lieu of the third 

 in the common erecting eye-pieces. Mr. W. Tulley has, I 

 believe, satisfied himself that this species of triple achromatic 

 in two .parts may be executed, though he has not yet sue- 



