HA RD IVICKE'S SCIENCE- G OS SIP. 



217 



THE TWO MIRRORS. 



By W. J. N. 



WISH to help some 

 of the younger mi- 

 croscopical readers 

 of Science-Gossip 

 to make a more in- 

 telligent, and there- 

 fore more success- 

 ful, use of the mir- 

 rors, for the purposes 

 of illumination. 



Illumination is to 

 every microscopist 

 a suhject of special 

 interest. Its im- 

 portance is so great 

 as to be quickly 

 recognized, even by 

 the young observer. 

 Its difficulties are 

 as quickly felt. 

 Yet it is, perhaps, 

 seldom studied with sufficient care and thoroughness, 

 owing, it may be, to a fear that the investigation 

 would prove both difficult and tedious ; that it would, 

 in fact, require more time, and a larger amount of 

 mathematical knowledge, than the young microscopist 

 has at command. 



But if the principles which underlie the art of 

 illumination be not understood, it naturally follows 

 that the art itself will remain unmastered, and that 

 the practical details will be the occasion of more or 

 less perplexity. Difficulties will be unexpectedly met 

 with, which the observer, failing to account for, 

 cannot with any certainty overcome. The results of 

 his most careful efforts will frequently be imperfect. 

 A good result, if obtained at all, will be obtained 

 rather by accident, than as the result of intelligent 

 design. The light is then sufficient in quantity, 

 without being in excess : is cool and soft in quality, 

 and admirably helpful of the powers of the object-glass. 

 The favourite slide receives a new dower of beauty. 

 The eye detects in it new delicacies of detail, and, 

 after a prolonged observation, experiences no fatigue. 

 But when, next evening, the object is to be exhib'ted 

 No. 262.— October 1S86. 



to a friend, under identical conditions of illumination, 

 how different the result ! The light is red and hot ; 

 or, its glare is blinding ; or, it is dim, and cannot be 

 made agreeably brilliant. The object-glass works 

 feebly. Details appear indistinct — confused. The 

 eyes smart and water under a strange feeling of strain. 

 What the error may be, the novice does not know. 

 He moves the mirror. The lamp and bull's-eye are 

 tried in new positions. But all ends, as it began, in 

 comparative failure. Any microscopist who consults his 

 memory, will be likely to recollect some annoying 

 incident of this kind. 



Illumination as a whole, is a large subject ; but 

 many of its special difficulties will be found to cluster 

 round a comparatively small part of it — namely, the 

 employment of reflected light ; the agent of reflection 

 being usually one of two mirrors attached to the 

 microscope. I propose to take these mirrors as a text, 

 but with liberty reserved to wander a little away from 

 and around it as occasion may require, yet always 

 within reasonable limits. 



At the outset, something must be assumed. I 

 suppose the reader to be the happy possessor of a 

 fairly good instrument, fitted with such simple 

 accessories as a diaphragm, and a spot-lens ; and 

 that he has a suitable lamp. The microscope will 

 have two mirrors ; one plane, the other concave. 

 These mirrors will possess certain powers of adjust- 

 ment. They may be turned aside, right or left, from 

 the central position, and may be placed at any desired 

 distance from- an object on the stage between the 

 limits of about two inches and four and a half 

 inches. 



It is quite likely that the young microscopist has 

 no clear ideas as to the need there is for so considerable 

 a range of adjustment ; nor as to the conditions under 

 which it would be desirable to place the concave 

 mirror at some one of the possible distances, say, 

 for instance, at two inches and four-fifths. It may 

 be that he could not state what is the distance between 

 the centre of his concave mirror and its principal 

 focus ; nor even explain the meaning of that ex- 

 pression. In such a case, an intelligent use of the 

 concave mirror is not to be supposed possible. 



