140 ANNUAL OF SCIENTIFIC DISCOVERY. 



more inflaming to the eyes, than the pure and cold natural product 

 of the caverns of the earth. An examination with the tourmaline 

 tongs, or "pebble trier," shows common glass to be almost dark, 

 where the rock crystal is luminous ; and, on touching them both 

 to the tongue, there is observable at once the most marked differ- 

 ence as to temperature. Brazilian pebble glasses cost the manu- 

 facturers 10 times as much as the ordinary glasses, yet the price 

 at which tliey are sold is only 4 times that of the latter, namely, 

 $6 and $li respectively. It is certainly unwise economy that 

 would submit so delicate an organ as the eye to the use of imper- 

 fect or irritating glasses. 



CEMENTATION. 



•' Under this head I desire to make a few remarks upon the 

 cementing together by a transparent substance of two surfaces of 

 glass, such as the component parts of an achromatic lens, or cov- 

 ering a transparency or opaltype with glass so that the two shall 

 be in optical contact, and the junction therefore invisible. The 

 advantages of cementing lenses are increased brilliancy and 

 rapidity from the destruction of two internal reflecting surfaces ; 

 among the advantages accruing from the cementation of transpar- 

 encies being perfect protection from the atmosphere, and in the 

 case of a painting, such as a slide for a lantern, greater transpar- 

 ency and beauty, arising from the brush marks and its other sur- 

 face irregularities in the paint no longer interfering with the 

 perfect transmission of the light. The best cement for eftecting 

 the union is Canada balsam, which, if too thick, should be thinned 

 with a little turpentine, benzole, or ether. It is of importance 

 that no air-bubbles be present, as they prove a source of annoy- 

 ance. Let it be assumed that the object to be cemented is a lens, 

 (and it may be observed that the instructions for effecting this 

 apply equally to a transparency or an opaltype). Having thor- 

 oughly cleaned the surfaces to be brought in contact, lay the flint 

 glass, previously made warm, down on a table suitably covered 

 to prevent the under surface from being scratched. Now, by 

 means of a peg of wood or otherwise, convey a drop of the bal- 

 sam to the centre of the lens, and then gently lower down upon 

 it the lens to be placed in contact, also previously made slightly 

 warm. Now appl}" a slight pressure, and the dark disk in the 

 centre, indicative of optical contact, will rapidly increase in size, 

 until at last the balsam reaches the marofin and begins to ooze out 

 at the edges, if the balsam be present in excess. By means of a 

 piece of soft string passed crosswise over the lenses tie the two 

 together, and place them in a stove, an oven, or before a fire, for 

 a short time, until the balsam at the edges shall have become 

 hard and dry. Let the string then be removed and the lens freed 

 from all external traces of balsam by means of benzole, ether, or 

 even old collodion. The above directions, modified to suit cir- 

 cumstances, apply to the cementation of transparencies or opal 

 pictures." — Humph-ey's Journal. 



