left forearm finds it a most convenient rest in working the fine 

 adjustment screw. In addition to the three sleeves already de- 

 scribed, the long girders carry two cross-.pieces for the attach- 

 ment of various accessories. These wooden cross-pieces slide 

 up and down and are weighted with sash weights so that their 

 adjustment may be quickly and easily accomplished. In order 

 that the friction on the girders may not cause any inconvenience, 

 arms extend upward from these cross-pieces for the purpose of 

 carrying pulleys which are in contact with the edges of the 

 girder and so reduce the friction. These cross-pieces are clamp- 

 ed in position by set-screws at the side. It will be at once evi- 

 dent that these cross-pieces may be used for the attachment 

 of a variety of accessories. Among the more important of these 

 is the microscope camera. See 19, Fig. i. This hangs above the 

 microscope and is ever in readiness for instant use. The camera 

 itself presents no very peculiar features. It is of course a vertical" 

 pattern carrying the exposed photographic plate in a horizontal 

 position. It cannot be used in a horizontal position. Experience 

 has s'hown that the vertical position 'has very many advantages, 

 and that if one is confined to a single outfit, the vertical outfit is 

 the better, providing its attachment can be of the nature here 

 described. In obtaining fhe focus, the cross-piece carrying the 

 camera is loosened by unclamping the side screws and is then 

 moved upward and downward against the sash weights which 

 counterbalance it. A scale is marked on the girders so that the 

 various magnifications are at once obtainable, or they may be ob- 

 tained by special measurement in each case. The apparatus never 

 needs any levelling, being, as before said, constantly ready for 

 use. The operator loosens two hooks and the camera drops 

 instantly into position. The whole is ready for use in a few 

 seconds' time. 'If the photograph is being taken with a high 

 power and the illumination is therefore weak, and the exposure 

 consequently long, one leaves his instrument during the expos- 

 ure with the greatest confidence that nothing can disturb it. 

 Any tremors in the building will not be received either by the 

 microscope or the photographic plate. 



A second attachment of great importance for the production 

 of illustrations is the 



CAMERA LUCIDA. 



This presents a number of peculiarities. Fig. i, 17, 24. The 

 history of the camera lucida is a very interesting one. It is impos- 

 sible to go into its details 'here, but nothing is clearer than that 

 this instrument is one of great importance to the microscopist, 

 and its history is in accordance with this fact. The utmost in- 



