PRACTICAL PHOTO-MICKOGKRAPHY. 159 



light are so small compared with those of oil that we recom- 

 mend the former in all cases where common house gas is 

 attainable. For any ordinary class rooms the light produced 

 by a " blow-through " blow-pipe on a " soft " lime will be found 

 ample, either for projecting a photographic slide image on a 

 12 to 15 feet screen, or an image of the actual object if suit- 

 able at all or a screen of about 5 or 6 feet diameter, using in 

 the latter case a microscopic objective, in the former a photo- 

 graphic lens of the portrait type. For our " blow-through " 

 jet we require one tube of the jet to be connected with the 

 gas supply of the building ; the oxygen gas we keep in a steel 

 cylinder under pressure, or in a bag between weighted pressure 

 boards ; these with a supply of lime cylinders are all the appli- 

 ances required. There is no smell, no cleaning needed, no 

 danger, nor any notable heat. There are several forms of 

 blow-through, or as it is sometimes called, " safety" jet. The 

 hydrogen issues and burns at the larger orifice, the oxygen 

 is forced at some pressure through the hydrogen flame, 

 a jet of the two gases burning impinges on the lime, which 

 like other refractory substances gives a brilliant light in com- 

 bustion. The area of incandescence with this jet is larger 

 considerably than the area produced by the "mixing-jet," 

 which gives a light both smaller and brighter. The " safety " 

 jet is on the whole better adapted for the lecture-room than 

 the mixing jet, not so much on the score of safety as on that 

 of convenience. A mixing jet is shown in Fig. 11. 



The objection usually raised in our hearing to the use of the 

 lantern for classes is the difficulty of darkening the lecture- 

 room. There need be no trouble on this point, though there 

 may be some expense if the lectures are given in the daytime. 

 Ordinary folding shutters, if decently fitted, will exclude light 

 sufficiently, but in Britain we have a very good shutter for any 

 purpose, probably the best for all purposes, called Clarke's 

 Patent Shutter. It is made on the "Louvre" principle of 

 strips of metal or wood, and on being raised it coils itself up 

 into a receptacle above the window. These shutters can be 

 pulled in a few seconds, and in the writer's house they shut 

 out every beam of light that could mar the lantern image. 



