496 Chronicles of Science. [July? 



in the glass, and to leave the internal diameter not less than 1 mm., 

 whilst at the extremities the tube is wider, so as to fit ordinary join- 

 ings. When the gas has passed through the tube for about ten 

 minutes, the joinings are loosened and each end stopped with a piece 

 of glass rod. The bulb tube thus closed is then removed from the 

 evoluting vessel, and the thinnest part of the tube brought some little 

 distance above a very small Bunsen's flame ; the glass then softens 

 below a red-heat, and the ends may be drawn out and sealed with 

 safety. It is advisable to number the bulbs, and to test the first and 

 last by exposing them to a strong light. Frequently, in spite of every 

 precaution, the gas explodes during the act of sealing, so that in this 

 operation it is advisable to hold the bulb with a cloth rather than in 

 the open hand. As soon as one bulb-tube is removed, another is 

 placed in connection with the evolution flask, and, after ten minutes, 

 sealed as described. The above quantity of acid will serve for the 

 production of sixty bulbs. Thus prepared, the sealed bulbs may be 

 kept in the dark for any length of time without injury ; some, which 

 were known to have been made more than a year, were found to be 

 perfectly good. To explode these bulbs it is only necessary to expose 

 them to diffuse daylight or sunlight, when the combination occurs in- 

 stantly. Of artificial lights, the bright flash produced by the com- 

 bustion of the vapour of bisulphide of carbon in nitric oxide is most 

 effective ; but the light of burning magnesium wire, of phosphorus in 

 oxygen, or the electric light, answers perfectly well. Professor 

 Roscoe stated that Mr. Dancer, of Cross Street, had undertaken to 

 supply the bulbs to persons unable to prepare them. 



At the April Meeting of the same Society, Mr Dancer brought 

 forward some curious experiments in pseudoscopic vision through 

 prisms. If we look with both eyes at an object, such as the flat top of 

 a table, for example, and then interpose a prism between one eye and 

 the object, we discover, after a short time, that the portion of the sur- 

 face to which the sight is particularly directed has apparently changed 

 its distance. If, in trying the experiment, the thin edge of the prism 

 is turned inwards to the nose, the flat surface will appear concave ; if, 

 on the contrary, the base or thick edge is turned towards the nose, the 

 surface will appear convex. The full effect of this alteration in the 

 appearance of the object is not realized immediately, some persons see 

 it perfectly in a few seconds, others require some moments of steady 

 gazing before it becomes evident to them. The character of the 

 surface to which the vision is directed exercises some influence in 

 producing the effect. A circular table covered with a cloth of a 

 bright pattern, having a few articles disposed towards the edges, ex- 

 hibits this fallacious vision in a marked degree. The angle of the 

 prisms for showing these experiments should be about 15° ; if less 

 than this, the elevation or depression of surface is not sufficient to 

 produce a good effect ; if the angle is much greater than 15°, many 

 persons are unable to unite the refracted image of the prisms with the 

 real image seen by the other eye. Achromatic prisms are much to be 

 preferred in these experiments to those which are uncorrected for 



