1 1 4 Mr. Coddington's Meply to Dr. Goring. 



4. " In order to form a truly achromatic erecting eye-piece, 

 there must be a compensation both in that part which erects 

 or forms the image, and in that which views it ; therefore no 

 achromatic erecting eye-piece can he made with so few as three 

 lenses." 



This is altogether incorrect both in theory and in practice. 

 In an erecting eye-piece the compensation of dispersion may 

 take place any how, provided only that it be completely ef- 

 fected on the whole. Erecting eye-pieces of three lenses are 

 not used, though they may be made perfectly achromatic, be- 

 cause they are liable to another defect which cannot be com- 

 pensated. 



5. " An erecting eye-piece can only be made really achro- 

 matic, (if we do not employ concaves of flint glass,) by com- 

 bining two Huygenian eye-pieces." 



Did this gentleman ever examine the day eye-piece of a 

 common hand telescope ? 



6. " Such an eye-piece could only be used for viewing an 

 image, and could never be employed as an engyscope, because 

 it would have no external focus in front of the bottom glass." 



This is like No. 4, inaccurate, both as to the fact and the 

 reason. Any erecting eye-piece may be used as a microscope, 

 though it would serve little purpose on account of the very 

 small magnifying power that can be thus obtained. Nobody 

 would, however, find fault with it on that account, any more 

 than he would condemn a good carving-knife, because it would 

 not shave him clean. The Doctor then observes, that I have 

 been at the pains of falsifying my own theories, practically at 

 least, to the best of my abilities, by presenting to opticians 

 a compound microscope, termed achromatic, which is con- 

 structed according to them. He says, " I assert point blank, 

 that his instrument is as complete a failure as anything of the 

 sort I ever attempted myself. I have examined one of these 

 instruments of the latest and most improved construction, made 

 by Mr. Gary, and can, I think, be positive that both the chro- 

 matic and spherical aberration of the objective part was wholly 

 untouched, and that the eye-piece, consisting of four glasses, 

 was achromatic." 



The history of this instrument is briefly this : On a critical 

 examination of all the four defects to which a compound mi- 

 croscope of the ordinary kind is liable, I judged, from theory, 

 aided in some little degree by my own experience and that of 

 others, that two might be wholly removed in an instrument of 

 small volume and price, and the other two so much reduced 

 as to produce a microscope which miglit prove very useful to 



naturalists 



i 



