DISCOVERY 



173 



photographic or other illustrations. The ordinary 

 magic lantern used at lectures and other demonstrations 

 requires darkness and specially prepared lantern slides, 

 but the " dussaud." as the inventor has elected to call 



his apparatus, is operated in the full light of day, and 

 is so constructed that any object of a size that can be 

 introduced into the apparatus can be projected on to 

 the screen or anpvhere in the room where the demon- 

 stration is taking place. This enables the members of 

 the audience to take notes and saves the great cost of 

 making lantern slides. 



Before giving a detailed description of the photo- 

 graphs which illustrate this article and the various 

 uses to which the " dussaud " can be put, let me state 

 the principles upon which the lantern is based, in 

 accordance with the official communication made to 

 the Academy of Sciences. 



The maximum of luminous efficiency has been 

 obtained by means of condensers with one or several 

 lenses, the focus of which has been calculated by 

 M. Dussaud in relation with that of the reflectors at 

 the back of the source of light, and in such a manner as 

 to make it as short as possible — in certain cases not 

 more than 5 centimetres. In this way the whole of the 

 light is concentrated by reflection on the object to be 

 lit up and projected. M. Dussaud claims — and the 

 demonstration before Parisian scientists proves his 

 contention — that his optical systems realise the con- 

 ditions of precision and yield of microscopes, and 

 constitute, in fact, collective microscopes. With half- 

 watt electric lamps, only 3 amperes from a voltage of 

 no volts (the minimum quantity of voltage supplied 

 by Parisian electric-light companies) suffice to project 

 objects with their form, colour, relief, and movements, 

 the dimensions of the projection being as large, if 

 necessary, as g square yards either on a screen or 



on any -vertical, horizontal, or oblique surface. The 

 apparatus is very easy to use, and its small size and 

 weight enable it to be held in the hands in all positions. 

 For home use it is admirable, since it can be connected 

 with the electric-light current by means of any ordinary 

 plug. The apparatus is cooled by circulation of air 

 around the lamp, the optical system, and the object 

 to be projected, the temperature not rising above 25° 

 Centigrade (77° Fahrenheit) after the apparatus has 

 been in use for a quarter of an hour. The object to 

 be projected is placed at the base of the apparatus, 

 which thus forms a sort of laboratorj' table fully lit 

 up and the dimensions of which, according to the 

 type of apparatus used, are 6 cm. by 6 cm., 12 cm. 

 by 12 cm., or 24 cm. by 24 cm. 



M. Dussaud has devoted seven years to the solution 

 of the problems which this apparatus set him. He was, 

 indeed, faced by a triple problem : a mathematical one 

 through the calculation of the curves, a chemical one 

 through the composition of the matter used, and a 

 physical one through the calculation of the indices of 

 refraction, " because he considered that such an optical 

 system, enabling one to see objects enlarged up to 

 125,000 times in volume, in a fully lighted room, would 

 render the greatest service to science." 



We will now examine, in the light of the illustrations, 

 the various uses to which the " dussaud " can be put. 



Fig. I represents its use with two cj'linders, on which 

 is unrolled and rolled a spool of paper bearing illustra- 

 tions or a text which have to be projected. The lamp 

 and its reflector is in the box on the right, and the light 

 is thrown on to the images as they are unrolled by the 

 hand of the lecturer. Above them is a combination of 



lenses which throws the pictures on to a mirror which 

 swings on an axis, and it is this mirror which projects 

 the illustrations on to the screen or wall or ceiling, 

 according to the angle at which the mirror is set. It is 

 because of this ingenious arrangement that projection 



