164 Mr. W. 11. Grove on Aplanatic Telescopes. 



pour the cement on to the centre of the convexity in the flint 

 and replace the crown on it, press with the finger on the centre 

 of the crown until the cement exudes all round from between 

 the glasses ; remove the lamp and support the plate of iron by 

 a firm prop ; then place a thick annular pad of filtering-paper, 

 two-thirds of the diameter of the glass, on the crown glass, and 

 on this a moderate weight, such as will press the glasses very 

 close without being so heavy as to distort them ; the weight had 

 better be as warm as the glasses. Props should be arranged so 

 as to keep the glasses in situ ; and if they move on each other, 

 a little adjustment may be made by the hands. Allow the whole 

 to cool slowly ; when cold, wash off the overflow of cement with 

 naphtha or other solvent, and the operation is complete. 



2nd. To filter rosin or similar substances when not quite clear. 

 — Procure the most transparent rosin ; I have obtained some at 

 drysalters' in Long Acre, which in films of 0*1 to 0*2 of an inch 

 has scarcely perceptible colour. 



Make a single conical filter of white filtering-paper, suspend 

 it by the ring of a retort-holder placed near the top, using no 

 external funnel; put the rosin, roughly broken, into this filter, 

 and place it a few inches in front of a bright parlour-fire, turn- 

 ing it occasionally : the rosin will melt and drop from the filter 

 beautifully clear. It may be collected in a vessel, or dropped at 

 once into castor-oil or other liquid used ; they should then be 

 warmed and well stirred together, and may be put aside for use 

 when required. 



3rd. Levelling. — If the glasses thus prepared have not the 

 meniscus of cement of uniform thickness for the same distances 

 from the centre (a defect ascertainable in a moment by looking 

 through the telescope at a real or artificial star), warm them again 

 until the cement is melted, and no more, and with the two hands 

 gently pinch the edges together, turning the glasses constantly 

 round until cold, or nearly so. I have never found this fail if 

 neatly executed. 



If air-bubbles appear in the cement, slide the one glass when 

 hot over the other until the bubbles escape, and then gently 

 slide it back. 



Many details can only be learned by practice. I have given 

 all the points as to materials and manipulation which occur 

 to me as essential. I strongly urged on Mr. Cooke some 

 years since to take up the subject, as I am, of course, not suffi- 

 ciently a practical optician to make good telescopes. I hope it 

 will now be taken up in earnest. 



January 26, 1867. 



