THE ROSS MODEL. 1 1 



the workmanship, depends the excellence of the work it is 

 able to perform. 



The two principal models, upon which nearly all stands 

 are made, have been named the " Ross model " and the 

 " Jackson model," both of which may be studied in the 

 productions of Messrs. Ross & Co. Fig. 5 shows the Ross 

 model, in which the body is attached by its base to a 

 transverse arm, which latter is borne on the summit of a 

 racked stem, to be seen in the figure. 



This is a very efficient and convenient form when the 

 workmanship is good, and leaves nothing to be desired 

 if the stem and transverse arm are sufficiently solid. At 

 the same time, it is the worst model on which to make 

 cheap instruments, as those who have used them will only 

 be too ready to testify. A cheap stand made on the Ross 

 model practically requires no fine adjustment, there being 

 sufficient spring in the stem and arm to focus with, by slightly 

 pressing upon the latter at its junction with the body. 



In the Jackson model, shown in Fig. 6, the body has the 

 rackwork attached to it, and is supported for a great part of 

 its length on a solid limb, as shown in the figure. It is, there- 

 fore, much less liable to vibration than in the Ross model. 



In a paper read before the Royal Microscopical Society 

 in March 1870, Dr. Carpenter detailed his experience of 

 these two forms of instruments. He sums up as follows : 

 " My own very decided conviction is that the adoption of 

 the principles of the Jackson model would be decidedly 

 advantageous alike for first-class instruments, in which the 

 steadiness of the image when the highest powers are being 

 employed ought to be of primary consideration ; for those 

 second-class instruments which are intended, at a less cost, 

 to do as much of the work of the first class as they can be 

 made to perform, portability here being of essential im- 

 portance; and for those third-class instruments in which 



