The Eye and the Microscope. 431 



gently moved backwards or forwards till the best condensa- 

 tion of light; is obtained. 



The lesson with the echinus* spine is not yet over. It 

 should be seen as an opaque body., with the bull's-eye lens, 

 with the lieberkuhn and dark well, and with the side silver re- 

 flector. If the bull's-eye is intended to give a strong spot of 

 light, its convex side may be turned towards the lamp ; but 

 when employed to increase the quantity of light which falls 

 upon the mirror, its flat side should occupy that position. The 

 lieberkuhn is very easily used, and if a few shillings do not 

 matter, it is better to get a set of dark wells from the optician 

 than to make shift with spots of velvet or lamp-black upon a 

 glass slide. The side silver reflector cannot be recommended 

 to a mere beginner, but after a few months work has given some 

 skill in manipulation, it is a valuable aid. It is best mounted 

 on a stand like a little bull's-eye, and should hare motions 

 in all directions, including the means of changing its height. 

 When it is to be used, place the lamp on the right hand, the 

 bull's-eye in front of it, and the silver reflector on the left, so 

 that it may catch the light, which will be thrown across the 

 stage. Thus arranged this piece of apparatus has several ad- 

 vantages. It gives light with one reflection from a single 

 surface, which has no tendency to become chromatic, and it 

 affords great facilities for oblique illumination. With such an 

 object as the elytron of the diamond beetle it has a gorgeous 

 effect, and, at -the same time, brings out fine lines which an 

 equal amount of light less judiciously applied would efface. It 

 will not do so well with the echinus' spine, as the method 

 of dark-ground illumination already described; but the student 

 should try this, and other objects, in every possible way, and 

 will thus acquire manipulative skill and judgment concerning 

 the best plan to employ. 



Having gone through a set of illuminating experiments 

 with the echinus' spine, repeat them with the slide of arach- 

 noidiscus, and specially note the beautiful appearance which 

 these diatoms present when the parabola is used. They 

 then resemble the exquisite filagree-work which the Maltese 

 execute in silver ; and if the parabola is suddenly removed, 

 and light from the mirror under the stage sent through the 

 object a great change will be noticed in the disposition of the 

 light, and dark lines. The markings of the arachnoidiscus must 

 be displayed as they are shown in good drawings whatever 

 mode of illumination be employed, and it is well to select a 

 slide containing, in addition to one or two arachnoidisci, some 

 other large diatoms, such as Triceratium, Biddulphia, etc., which 

 can be displayed with the same power. 



The objects recommended for the first lesson in such a use 



