1886-87.] Hiuncs Projection Microscope. 63 



the two great essentials are steadiness of the whole, so that any 

 adjusting or focussing does not produce a visible movement on 

 the screen ; and also, it must be capable of easy and rapid ad- 

 justment, and of picking out on the slide any part desired to be 

 brought to the centre. In the lighting arrangements, also, 

 steadiness and freedom from flickering, as well as easy man- 

 ipulation, are essentials. In the apparatus before you these 

 requirements are obtained in a very high degree, as I shall 

 show you afterwards. These being the general principles essen- 

 tial to successful projection, it remains for me to describe to 

 you Mr Hume's apparatus for obtaining them. In it the lime- 

 light is used by means of the mixing burner, having a power 

 of about 500 candles. This is fitted with an ordinary lime- 

 turning arrangement, and has adjustments for bringing the 

 light to the optical centre. This is contained in a mahogany 

 body, iron-lined, with air- passages to keep it cool. In front of 

 this burner is a 4-inch double condenser, consisting of a men- 

 iscus with the concave side towards the light, and a plano- 

 convex with the convex side to the light. This combination 

 has a back focus of about 2 inches, and has the light set about 

 1\ inches from it, giving slightly convergent rays. Another 

 single 3^-inch condenser, double convex, of 4f-inch focus, 

 which is capable of adjustment, slips in a tube before this. 

 All the rays emitted from the first condenser are taken up by 

 this, and are brought to a focussing-point at 4 inches from the 

 lens, where the disc of light is about i-inch diameter. At this 

 point the slide is introduced, where it has all the rays of light 

 converged on it. This arrangement of condensers is used for 

 low powers, stops being introduced for special objects to 

 sharpen the disc. Tor the higher powers a third condenser is 

 introduced, plano-convex, If-inch diameter and 1-inch focus. 

 This possesses the function of an Abbe's illuminator, bringing 

 the rays to a very bright spot about \ inch from the front of 

 it. The object is held by means of clips in front of this lens, 

 or the stops, on a steady mechanical stage, with vertical and 

 horizontal movements. This is mounted on a tube, within 

 which is carried the objective, which is held in a fitting cap- 

 able of being pulled out for the insertion of other objectives. 

 The whole is fitted with rack-and-pinion for quick focussing, and 

 micrometer adjusting-screw, one complete turn of which moves 



