HARDWICKE'S SCIENCE-GOSSIP. 



lamp-flame is not a small spot of light like the focal 

 image formed on the wall, but a group of spots massed 

 together. Only one small portion of the flame, 

 therefore, can at one time represent the focal image, 

 and emit rays which will all (apart from the effects of 

 aberration) be rendered parallel by the bull's-eye. 



We may, to a great extent, overcome these 

 difficulties thus : — Turn down the lamp-wick, until 

 the whole flame is very small indeed, and let one 

 end of this small flame face the mirror. This will 

 do much towards obtaining the small spot of 

 brilliant white light desired. Then, to get rid of 

 such admixture of convergent and divergent rays 

 with the parallel pencil, as is still unavoidable from 

 both the causes named, move the lamp, if possible, 

 to a con siderable distance from the mirror without 

 altering its angular position. Many of the non- 

 parallel rays will then pass outside the mirror, and 

 the intensity of the others will be so reduced as to 

 render them comparatively harmless. 



But it may be objected, that it would not be 

 possible to get sufficient light from so small a flame, 

 placed at a distance from the microscope. I reply, 

 that for most purposes (always excluding dark 

 ground) the illumination lamp-wick may be turned 

 down until the top of the flame is only just level with 

 the top of the brass burner, and the lamp may, in 

 addition, be placed at a distance of (say) two feet 

 from the mirror — and yet the light will be ample for 

 the one-eighth inch object-glass, plus the B eye-piece 

 (say for a power of 600 diameters), and will be of the 

 coolest, purest, kind — very agreeable to the eyes, and 

 admirably fitted to secure the perfect working of the 

 object-glass. I have found it possible to get a 

 middling view of diatoms under the f-inch, by 

 the light of a composite candle ; and since writing 

 the above paragraph I have examined many diatoms 

 under the one-eighth, with the light of a lamp that 

 was turned down as described above ; so that, in 

 fact, it appeared to be almost " out." 



Let the bull's-eye then be placed at the distance of 

 one of its principal foci from the nearest part of the 

 lamp-flame, the_said distance depending, as we have 

 seen, on which of its two surfaces faces the lamp. 

 Adjust its height carefully to that of the flame, so 

 that the rays may be received from the brightest 

 portion of the same, and that the centre of the pencil 

 may be thrown on to the centre of the mirror. If a 

 line be supposed to pass from the edge of the flame 

 to the centre of the mirror, that line should pass 

 through the centre of the bull's-eye at right angles to 

 its plane surface. (A piece of paper laid on the 

 mirror may assist the novice to direct the rays thereto.) 

 In getting the microscope and its accessories ready 

 for work, it is well to follow some regular method. 

 To the beginner, I will venture to suggest the 

 following : 



1. Decide what kind of illumination will best suit 

 the objects you propose to examine. 



2. Set the mirror at the proper distance from the 

 stage for the kind of illumination which you have 

 selected. 



3. Place the microscope on its platform, and by 

 means of the thread and buttons, set the tube nt the 

 proper angle of slope. 



4. If eye-piece or object-glass be present, remove it. 



5. Place the mirror in the axis of the microscope. 

 To do this, first glance down the outside of the tube, 

 and see that the movable stem on which the mirror 

 slides is in line with the tube itself. Then look down 

 the empty tube, and, without moving the stem, turn 

 up the brass hoop which holds the mirrors, edgeways 

 towards the eye, so that the mirrors face right and 

 left and are hidden from view. If the hoop is not 

 seen exactly in the centre of the field, turn the clip 

 round upon the stem till the centering is accurate. 



6. Slide the instrument, forward or backward, 

 until the centre of the mirror is directly over the spot 

 marked on the tray. 



7- Place the lamp alongside the mirror, and raise 

 it on its pillar (if necessary) until the top of the wick 

 is just below the centre of the mirror ; then remove 

 it to its proper place on the tray, which place must 

 correspond with the focal distance already assigned 

 to the mirror. 



8. Place the bull's-eye in position. 



9. Look down the empty tube of the microscope, 

 and, without altering any adjustment already made, 

 turn up the plane mirror till you can see in it the 

 illuminated bull's-eye. If the latter be in correct 

 position, the centre and an elliptical space around it, 

 but not the margin, will be brilliantly illuminated by 

 a whitish-yellow (not red) light, without any appear- 

 ance of dark spots. If the light be dim, or red, the 

 bull's-eye is wrongly adjusted in height. If the shape 

 of the illuminated surface be not symmetrical, the 

 bull's-eye is not truly facing the mirror. If the 

 illuminated surface have dark spots on it, or extend 

 to the margin of the lens, the incident rays are not 

 all parallel, the lens being too near the lamp. If 

 the illuminated surface be small, the distance from 

 the lamp is too great. No pains must be spared to 

 get the correct pencil. Failing here, you fail en- 

 tirely. 



10. If the diaphragm is to be made use of, now 

 slip its holder into position. 



11. Gently turn over the mirrors in their Y -holder, 

 that the concave mirror may be substituted for the 

 plane one. (For low powers the plane mirror is 

 better.) 



12. Place a trial-slip on the stage, and see whether 

 the light thrown upon it from the mirror is well- 

 focused and good. If not, some mistake has been 

 made, and must be corrected. 



13. Attach object-glass and eye-piece. 



14. Replace the trial-slip by the object, and rotate 

 the diaphragm till the pleasantest amount of light is 

 obtained. 



