Aplanafic Object-glasses. 251 



I must here state, that Messrs. C 's object-glasses are ali 

 stuck together, I beUeve, with fused gum-mastic, or, perhaps, 

 with very thick mastic varnish. This practice seems, in theory, 

 to be bad, most especially if the curves united together are not 

 of the same radius ; nevertheless, practically speaking, the 

 process of soldering seems to me to do more good, by thei obli-« 

 teration of two surfaces, and by keeping the glasses immov-* 

 ably adjusted, than harm in any other way. I cannot, in fact, 

 discover any very sensible difference in the optical performance 

 of these small achromatics, whether stuck together or not. I 

 fancy that they have a little more light and clearness when 

 cemented, (as they certainly should have,) but cannot be very 

 positive. I hold it as a maxim in practical optics, as in our 

 common law, ** de rebus non apparentibus etaon existentibus 

 eadem est ratio." 



I may observe, that Mr. Lister has combined that marked 

 10, with 4, and finds the performance proportional to that of 

 4 and 2. 



It will be remarked that Messrs. C, from an apparent igno* 

 ranee of the value of aperture, and perhaps impressed with the 

 too common and prevalent idea, that, having once obtained 

 distinctness and achromatism in their object-glasses, every 

 thing else might be accomplished by a condensation of artificial 

 light, have reduced their apertures to such a degree, tis to 

 render their instrument as ineffective upon test-objects as a 

 common compound ; for when the opening of an aplanatic glass 

 is cut off to the same diameter as a common one, it shows 

 nothing more, though it will certainly exhibit objects far more 

 satisfactorily. Upon the apertures of microscopic lenses their 

 effects entirely depend, as was remarked a long time ago by the 

 great Huygens. An achromatic glass is more valuable thaii 

 another, merely on account of the larger aperture it will bear< 

 without causing aberration, and consequent indistinctness. 

 Those who are in possession of Messrs. C.'s microscopes should 

 get the stop behind the object-glasses turned out, and procure 

 others to be used ad libitum, according to what the goodness 

 of the object-glasses will permit. 



I feel myself called upon, however, to state, that, since th^ 

 completion of Mr. L.'s microscope, Messrs. C. have enlarged th^ 



