IQQ QUERIES ON THE ARTS. 



times that of the second, and their distance f the focal length 

 of the first. I must own I do not see the reason why this com- 

 bination should remove the chromatic aberration, nor does the 

 author give any. He hints also how much superior such combi- 

 nations of lenses are for magnifiers of solar microscopes ; if this 

 is the case, why will not a similar combination answer for the 

 object-glass of a telescope ? 



That a combination of lenses may diminish or nearly remove 

 the error arising from the spherical figure, I can easily con- 

 ceivcj but not the chromatic aberration. 



JUVENIS. 

 June2d,lS06. 



The above questions may, and to you I dare say will, aj)- 

 pear trifling and frivolous ; but you will pardon them when you 

 reflect how frequently some little obstacle starts up to stop the 

 course of the young and ardent enquirer, and that unless a 

 kind friend remove it, seems an insurmountable barrier. 



Singular fact I vvish it were in my power to give full information concern- 



respecting the jjjf, ^Yid obiects of tlie present letter,. What it may be that in- 

 incrustation in " . „ ^ , , -, ■ ^ m 



fi kettle. crusts the surface of tea-kettles, and is very troublesome in 



steam engines, I do not know, but suppose it to be sulphate 

 of lime. Many years age I was informed, that if an oyster- 

 shell be constantly kept in a tea-kettle there will be no incrus^ 

 tation formed, except upon the shell. If this be true, the same 

 effect would be produced by a loose piece of incrustation, pro- 

 vided it were put in before any deposition had began to fix upon 

 the metal. 

 Porosity of 2. It does not seem probable that a bad speculum which 



gp'.'culum me- ^^^ j^^ pores discernible by the microscope owes its badness t6 

 porosity. When I was formerly busied upon speculums, it 

 was my custom to grind oflF a small portion of the metal and 

 polish it, in order to determine whether I should bestow an 

 farther labour upon the piece. This polish was often, for the 

 sake of expedition, given with a leather or buff stick ; and it 

 i-jpeculum always happened that the surface took a wavy appearance ; 

 "avyS^polish-an effect which did not follow when pitch was used as the po- 

 ed on leather. l}sher. I ascribed this irregularity to some parts being softer 



than 



