ACCESSORY INSTRUMENTS 149 



Certain point, however, they are not formidable, and we will 

 mention here some simple problems and forms of apparatus. 



Fig. 79 represents the best form of the simple and old- 

 fashioned microscopic attachment, which has been furnished 

 by all opticians for many years. A still commoner form is 

 sold, with only one slide-stage instead of the two here figured. 

 In either case, the ordinary objective is unscrewed from the 

 lantern front, and this attachment screwed in its place. It is 

 usually furnished with two objectives, each composed of a 

 couple of plano-convex lenses with their convex sides together, 

 stopped down in front ; but I have known two meniscus lenses 

 used with their convex sides to the object ; 

 and there is no doubt that with care in select- 

 ing the curves, and in proper spacing apart 

 of the lenses, these simple objectives will do 

 very fair work. I used a 2-inch focus lens of 

 this kind for some time for its large field, even 

 in the instrument hereafter described, with 

 good results. Generally the foci supplied range 

 from 2| and 1 inches, to 2 inches and 1 inch. 

 These objectives slide into the rack-front, and 

 thus roughly adjust focus, precisely as the 

 lantern-front recommended on p. 83, the higher 

 power having of course to be pushed farther into the tube. 



With the single-stage form of attachment, all that can be 

 done is to roughly focus the object on the screen, and then 

 draw back the lime-tray a little, and adjust it to and fro in the 

 lantern, till the object is most brilliantly illuminated, which 

 will be when it is just at the focus of the cone of rays from the 

 condensers. This will also, for reasons already pointed out, 

 be the position of best definition. The object may, however, 

 be rather too large to be covered by such a focus, and if so the 

 exhibitor must alter the focus a little to get the best result he 

 can. These results, with a good operator and a good jet, may 

 be very fair up to the limits of the powers supplied, which are 



