428 Mr. Coddington on the Improvement of the Microscope. 



shorter focal lengths, regard being had, in each case, to the 

 proper condition of achromatism. Thus several different eye- 

 pieces might be inserted, at pleasure, into one tube, in the same 

 manner as it is usual to vary the magnifying power of a telescope*. 

 I have not yet tried the eifect of this, but I suppose it may be 

 necessary in applying the microscope to opaque objects, as the 

 difficulty of illuminating them almo.st precludes the use of a 

 powerful object-glass. 



I do not pretend to give this as a perfect instrument — much 

 less as one that will answer all purposes; but having tried it, 

 in a very rough state, and with a moderate magnifying power, 

 on various delicate test objects, all of which it shows very satis- 

 factorily f, not excepting the striae on the scales of the Podura, 

 which Mr. Pritchard, the inventor of the diamond and sapphire 

 lenses, says are only just discernible with the most perfect in- 

 struments, I see no reason to doubt that, when carefully executed, 

 it will be found very effective, and that the naturalist may be 

 furnished, at an expense not exceeding five or six guineas, with 

 a microscope which will perform nearly all that can be expected 

 from that instrument. 



Fig. 3. represents the microscope, as I have directed it to be 

 made by Mr. Cary. 



* For example, if two equal plano-convex lenses having a joint focal length of 

 half an inch, be placed at a distance of two inches and a half from the centre of the 

 object-glass, and the eye-glasses be like those of the former eye-piece, but of focal 

 lengths half an inch and one-third of an inch, the distance between the two pairs 

 being ^in. the power of the object-glass will be multiplied about eight times. 



t Mr. R. Brown's active molecules may be very pleasantly observed with this microscope, 

 with a power of about 360. 



H. CODDINGTON. 



Trinity College, 

 April 23, 1830. 



