ILLUMINANTS 3 



existent. One disadvantage, however, putting aside the difficulties of its production, 

 maintenance, let alone its expense, outweighs all the advantages of its use. It arises 

 from the fact that the point of light is always shifting : however slight, still it is 

 frequently changing its position, so that the centring of the light with the optical 

 axis of the microscope is frequently getting out of adjustment, and pictures with 

 unequal illumination may as frequently result. In saying this we are fully aware 

 that some photo -micrographers still champion its use, but Ave are equally cognisant of 

 the fact that many have relinquished it on this very ground. 



Still we feel bound to admit that the steadiness of the arc light has of late been 

 much improved, and many lamps are now in the market of undoubted merit, to wit, 

 one sold by Messrs. Newton & Co., Fleet Street, and another, which is perhaps better 

 suited still, introduced by Ross & Co. quite recently. 



Lime-light, however, is that mostly employed. It is used in two forms : the mixed 

 jet, where the gases, oxygen and coal gas (or hydrogen), both under pressure controlled 

 by regulators to be presently described, mix in the jet itself just before ignition ; and 

 the blow-through form, which may often advantageously be employed for low-power 

 work, when the oxygen under pressure blows through and into a stream of house gas 

 issuing from an independent source, such as from the ordinary house tap, from which 

 it is led to the jet by means of a piece of rubber tubing. 



The Mixed Jet. — There are several forms of " mixed gas " jet, but space will not 

 allow us a description of them all ; indeed, it does not seem to be necessary, as the 

 same idea pervades the whole of them, and differences that do exist are those of 

 minute detail rather than those of difference of construction. Jets do differ very 

 much indeed from one another in performance, and we only mention some of those 

 with which we are familiar. 



Messrs. Newton & Co., of Fleet Street, the well-known opticians, sell an excep- 

 tionally fine jet designed by Mr. Lewis Wright, whose name, as associated with 

 optical projection, is a household word. It has an ingenious means of cutting off the 

 gases, we believe originally suggested by Mr. Pringle, of photo-micrographical fame. 

 It is shown in Fig. i , where not only is the jet itself figured, but also an improved 

 form of stand, quite lately introduced, which permits of lateral and up-and-down 

 movement, both governed by milled-headed screws. This is of great service to the 

 photo-micrographer as well as the ordinary lanternist. But the jet we have ourselves 

 mostly used with so much satisfaction is the high-power form designed by Beard. It 

 is shown in Fig. 2 and also in Fig. 3, where it is seen dropped over a pin which 

 is attached to the special stand of our own design which enables the operator to 



