132 TKANSACTIONS OF THE 



microscope body and can be swung round for the purpose of bring- 

 ing rapidly another object-glass in position. The double nose-piece 

 is handier than the triple or quadi-uple one, but only the bent 

 form as devised by Powell and Lealand should be bought, as it is 

 the safest, carrying the objectives which are not in use entirely off 

 the stage. 



8. The camera lucida, a small instrument fitting over the eye- 

 piece and reflecting the image on a sheet of paper on the table 

 when the microscope is placed horizontally, so that the outlines of 

 an image can easily be traced and measured. The observer must 

 look with the upper half of the pupil of one of his eyes on the 

 prism or on the steel or glass disc, and with the lower half of the 

 pupil he requires to follow the pencil point. The illumination 

 must not be too bright, else the pencil point is hardly seen. The 

 best form is Wollaston's prism camera-lucida. To measure the 

 magnification a micrometer, i.e., a slip of glass, divided into hun- 

 dredths, thousandths, or ten-thousandths of an inch, must be 

 placed on the stage and focussed, and the magnified image of these 

 divisions traced by means of the camera and measured. 



9. The eye-piece micrometer for measuring objects in the micro- 

 scope consists of a micrometer either permanently or temporarily 

 fixed between the field and the eye-glass of the eye-piece, and after 

 the value of its divisions has been ascertained by means of a stage 

 micrometer, the sizes of objects under observation may easily be 

 read ofi". Jackson's micrometer is a scale on glass in a brass frame 

 which can be inserted into the eye-piece, and which is further 

 adjustable by means of a very finely divided screw. Hamsden's 

 screw micrometer is the most perfect apparatus for measuring the 

 size of objects in the microscope, and consists of an eye-piece 

 containing two parallel thin wires which can be separated or 

 approached by means of a screw. Up to the 1-75, 000th of an 

 inch can be measured by means of this instrument. 



The simplest way of measuring roughly the size of the magnified 

 image is to look down the microscope with one eye and with the 

 other eye to look on a scale held close to the body of the instru- 

 ment at a distance of 10 inches from the eye. In this position the 

 observer sees both object and scale simultaneously, and can read 

 off the size with tolerable accuracy. 



10. The eye-piece goniometer serves for measuring the angles of 

 crystals, &c., by means of a scale in the eye-piece, which can be 



