246 Dr Goring' s Remarks on M. Chevalier's Paper, 



I suppose, if it is conceded that the object-glass of a small 

 telescope should, when turned the wrong way, constitute that 

 of an engyscope, the object-glass of the latter must also, when 

 turned the wrong way, form that of a telescope. Now we 

 will screw this doctrine up to the sticking-place, and see if it 

 will be found wanting or not when made to stand the brunt 

 of facts. 



The following are the results of trials with a T ^th triple en- 

 gyscopk object-glass used as a teleseope. I must premise, 

 that the said object-glass is achromatic and aplanatic, with an 

 aperture of about f v of an inch, used as a microscope, but if 

 the aperture is greater, the spherical aberration begins to pre- 

 ponderate in the covcare. If the whole aperture t % is em- 

 ployed, it is very violent in the concave, and there is also a 

 tendency to over-correction in point of colour. I mounted 

 the said object-glass as a telescope, with an Hnygenian eye- 

 piece, which made it magnify nearly four limes, and reversed 

 the order of its lenses. 



Whole aperture. — Spherical aberration very violent in the 

 c&rivex. There seems to be no difference which way it is turn- 

 ed towards the object. 



Cut off to j\, do. do. 



Cut off to j 3 5 , the same but weaker. 



Cut off to y\j, spherical aberration, the same way as before, 

 whether the object-glass is inverted or not. A little under cor- 

 rection in point of colour, now becomes manifest. This was be- 

 fore corrected by the strong spherical aberration in the convex 

 lenses, which is now, by the reduction of the aperture, consi- 

 derably weakened in intensity- 

 Aperture j T 5 . — Aberration greatly reduced, but still the 

 same way as before ; that of a plano-convex lens of the same 

 focus and aperture, with its covered side towards the object, 

 scarce to be distinguished now from that of the achromatic. 



I may observe, that all the triple achromatics I have ever 

 tried give much the same results. The curves of the T ] n ob- 

 |i c t -glass are, 



Radius of 1st surface, 0.825) /r , . 



2( |, 0.525) ^vex, (Crown) 



