170 



OPTICAL PROJECTION 



upwards. An adjustable mirror can then be attached by a 

 very simple fitting, and will reflect the image upon the screen. 

 The objectives of both lanterns are mounted on the detached 

 system already described for physical work. One lantern is 

 therefore always ready for ordinary work of any kind, while 

 the top one can either be used in the same way (the whole 

 as an ordinary bi-unial exhibition lantern if necessary) or can 

 be converted at a moment's notice to vertical purposes. A 

 screw is provided to keep the tray and jet from dropping out 

 of the proper position. 



I was at first apprehensive that the heat of the jet, when 

 thus brought under the condenser, might prove a fatal objec- 

 tion to this arrangement ; but it has not proved so, provided 

 the jet be kept in work while the lantern remains vertical. 

 If the jet is turned off so that the flame of coal-gas, released 

 from the pressure driving it down, rises upwards towards the 

 condenser, a crack will probably result; but it is easily 

 avoided with this caution, and more brilliant projections are 

 thus obtained than in the other way. 



86. Erecting Prism. For the mass of physical experiments 

 the ordinary inversion of an image does not signify, and is 



readily understood by the 

 audience. For thermometric 

 and some other experiments, 

 however, it becomes of im- 

 portance, and in such cases 

 is obviated by the use of a 

 prism, as described by Bertin, 

 Miiller, and others. Fig. 93 

 shows the action of a right- 

 angled prism P used in this way. The ray a b is refracted 

 to c and there reflected, and again refracted as de\ while 

 fg is refracted to h and there reflected, coming out as i k ; 

 thus the top and bottom rays are inverted, and the image is 

 erected. The best position for the prism is where the cone 



FIG. 93 Erecting Prism 



