38 Dr Goring on Reflecting Microscopes. 



those made by Mr Pritchard), would also do if very perfect* 

 The total length of this eyepiece will be about 6 inches, and it 

 should be so contrived, that its anterior conjugate focus should 

 fall at I5 inch in front of the achromatic glass or glasses. 

 The diameter of the tube of this eyepiece must not exceed f 

 inch, but its objective end must, for the last 1^ inch, be taper- 

 ed, so that the diameter, at the termination where the achroma- 

 tics are, shall be only half an inch. I had nearly forgotten to 

 mention, that a stop of the same diameter as the image of the 

 large metal formed by the achromatics, (about fths of an inch in 

 aperture probably), must be placed in the inside of the tube 

 behind the achromatics, and in their focus, for the usual pur- 

 pose of keeping out false light. The exterior of the said tube 

 must be carefully blacked, and it might not be amiss to cut 

 the whole of its surface into a screw with a fine tool, to arrest 

 and absorb light more effectually. The power of the instru- 

 ment will be determined by the depth of the Huyghenian eye- 

 glasses attached to the eyepiece, which must have eyeholes like 

 those of Gregorian telescopes, very accurately adjusted to the 

 size and locality of the visual pencil ; a mark should be made 

 on the side of the tube which holds them, and a corresponding 

 one on the exterior one, in order that they may be always in- 

 serted exactly in the same place, lest any inaccuracy of turning 

 should cause the eyehole to shift a little, and so let in false 

 lio-ht. The power employed must not, I think, be less than 

 thirty times, that is, the visual pencil must not be larger than 

 one-tenth of an inch, (which will cause the blot in its centre to 

 be somewhat less than one-thirtieth of an inch), otherwise, the 

 said blot will be apt to cause a faint luminous spot in the mid- 

 dle of the field of view ; the power may range from 30 to 60 

 to suit different states of the atmosphere, and near or distant 

 objects, — (on the former, a higher power may generally be us- 

 ed than on the latter, because there will be less air to look 

 through). It \vi\\ be necessary to attend to the foUowng cir- 

 cumstances in attaching the eyepiece to the instrument. I sup- 



• The said achromatics will be absolutelj' indispensable on account of the 

 large angle of aperture of the eyepiece (about 20°) ; it would have been 

 useless to have attempted the construction of a telescope of this kind with- 

 out them. 



