Philosophical Transactions. 131 



observations are limited in this paper, which is dated from 

 Ceylon : the former inhabits the lake of Colombo ; the latter 

 abounds in the houses and streets of the Pettah. The specific 

 gravity of the urine of the bull-frog is about 1003, and common 

 salt, with a little phosphate of lime, and a trace of urea, are 

 the only ingredients detected in it. The specific gravity of the 

 urine of the brown-toad is 1 008 ; its constituents similar to 

 the above, but the proportion of urea more considerable. For 

 the anatomical facts we must refer to the paper itself; the fol- 

 lowing physiological observations are important in relation to 

 the treatment of urinary disorders ; as such we quote them at 

 length. 



" Perhaps additional facts are not required to prove, that the secretion 

 of the kidneys of animals depends more on the intimate and invisible 

 structure of these organs, than on the kind of food the animals consume ; 

 were such facts wanting, there would be no difficulty in furnishing them. 

 How difl'erent is the urine of the brown-toad and that of any species of 

 small lizards ! yet flies are the favourite and common diet of both animals. 

 Other remarkable instances might be mentioned, of similarity of diet and 

 difference of urinary secretion ; and, vice versd, instances might be afforded 

 of diiiereuce of diet and similarity of urine : 1 will mention one only ; it 

 is that of parrots and snakes ; their urine, as I have found, being much the 

 same, consisting chiefly of uric acid, though tlieir diet is altogether differ- 

 ent, the birds feeding entirely on vegetable matter, and the reptiles en- 

 tirely on animal matter. But let me not be supposed to maintain that the 

 urinary secretion depends entirely on the organ, quite independent of the 

 nature of the food or of the blood, from which the elements of the urine are 

 derived. It appears to be pretty satisfactorily proved, that cctteris paribus, 

 there is a certain relation between the nature of the food and of the urine. 

 Whilst this has been general'y admitted, the relation between the organ 

 and the secretion has been less insisted on, though perhaps not less curious 

 and deserving of attention." 



ix. An Account of a Micrometer made of Rock Crystal. By 

 G. DOLLOND, F.R.S. 



Rock crystal has been applied to telescopes in various ways 

 for the purposes of micrometrical measurements : Mr. Dollond's 

 improvement consists in making a sphere or lens from a piece 

 of rock crystal, and adapting it to a telescope in the place of 

 the usual eye-glass, and from its natural double refracting pro- 

 perty rendering it useful as a micrometer. 



The advantages of this application of the crystal consist in 

 the saving of the time required to find the proper angle for 

 cutting it, and working its surfaces with sufficient accuracy ; 

 in being able to take the angle on each side zero, without re- 

 versing the eye-tube, and also to take intermediate angles 



the mere purpose, as we presume, of laying claim to the merit of future 

 discoveries, to which they are less entitled than the Marquis of \Vorcesler 

 to the invention of the steam-engine, or Roger Bacon to that of gun- 

 powder. 



K2 



