Ghservatlons on the Camera Obscura, &<^. iil 



emploved it, in extracting this acid from animal fluids j it 

 gives the clearest distinction between the lactic acid and 

 the acetic. 



The lactate of iron is of a red brown colour, docs not 

 erystaljize, and is not soluble in alcohol. The lactate of 

 zinc crystallizes. Both the^e metals are dissolved by the 

 lactic acid, with an extrication ot hydrogen gas. The lac- 

 tale of copper, according to its difiereiit degrees of satura- 

 tion, varies Irom blue to green and dark blue. It does not 

 crystallize. 



It is only necessary to compare the descriptions of these 

 salts with what we know of the salts which are formed 

 with the same bases by other acids, for example, the acetic, 

 the malic, and others, in order to be completely convinced 

 that the lactic acid must be a peculiar acid, peribcily distinct 

 from all others. 



XXX\' II] .CrilicalObservalions 071 Dr.WoLLASTON's stated 

 Improvement of the Camera Ohscura and Microscope in 

 the Application of the Meniscus and tivo Plano-convex 

 Lenses^ proving their Inferiority to the double Convex 

 Lens generally used. By William Jones, Optician, 



To Mr. Tilloch. 



Sir, — In your impartial Journal, vol. xvii. and also in 

 another cotempoiary Journal *, some observations of mine 

 were published, proving satisfactorily, I trust, that th« 

 periscopic spectacle glass advertised by Dr. Wollastou 

 as possessing a new optical principle, and affording an 

 improvement in the fiuure of a spectacle glass, wa< 

 no other than the old rejected meniscus lens ; con- 

 tained no refractive property different from tht plano-con- 

 vex and double convex lenses ; but, as it caused a greater 

 degree of aberration than those two lenses, was a worse form 

 of lens for spectacles or any other instrument than th» 

 double convex lens generally used by practical opticians;— 

 it must, therefore, surprise others conversant in optics, be- 

 sides myself, that Dr. Wollaston should be induced again 

 to propose the meniscus in the camera ohscura instead of 

 the double convex lens ; his accoym of which is copied 

 into your journal of last month from the Philosophical 

 Transactions for 1812. 



The desire that I have to maintain an optical truth, and 

 tbe duty 1 owe to our professional interests^ oblige me to. 



• Nicholson's Journal, vol, vii, 



Q 4 ' point 



