248 THE AMERICAN MONTHLY [November, 



engraver to correct the inaccuracies of your di'awings, and to complete 

 them in respect to their details ; but as a discipline for accurate obser- 

 vation, which is the first qualification of a microscopist, photo-micro- 

 graphy is inferior to drawing, and cannot be regarded as a substitute 

 for it. I would therefore urge you to endeavor to draw as many ob- 

 jects as you conveniently can, the records you will thus secure are of 

 far more practical value than many wordy descriptions. 



In order to draw and measure microscopic objects, and to determine 

 the magnifj'ing power of a microscope, you need the following acces- 

 sories : 



(i) A camera lucida or some form of reflector. I use Beale's. 

 (3) A stage micrometer, ruled to y^^ths and yJ^^ths of an inch. 

 Many prefer the millimetre scale divided into looths and loooths ; and 

 there is much to commend this standard, which is meeting with in- 

 creasing favor amongst microscopists, and indeed generally in all 

 branches of science. (3) A foot rule with a scale divided into tenths 

 of an inch ; (4) and a pair of fine-pointed compasses. 



The form of camera selected is not of much moment, and to some 

 extent will depend on the nature of the work to be done. Objects, e. g., 

 in fluids which have to be drawn with the stage in the horizontal posi- 

 tion will need a vertical camera, /. e., a special form which can be 

 used with the microscope in a perfectly upright position. The opti- 

 cians ofler several forms of this accessory. Mounted objects, however, 

 and indeed many in fluids, with ordinary care and patience, are usually 

 drawn with the body of the microscope laid in the horizontal. Which- 

 ever form of camera you select, be prepared to persevere in its use. 

 Beale's neutral-tinted glass, which costs six or eight shillings, is a small 

 disc of glass fitted in a light frame in which it is fixed at an angle of 

 4^"^ to the axis of the microscope. The principle of its construction is 

 a simple application- of the law of reflection- — that the angle of inci- 

 dence is equal to the angle of reflection. 



The micrometer is a slip of glass with a scale ruled on it, giving 

 divisions sufficiently fine to be available for ascertaining the very minute 

 dimensions of microscopic objects. Micrometers ruled to the milli- 

 metre scale are procurable. It will be suflicient if we confine ourselves 

 to the English scale for looths and lOOOths of an inch. The microm- 

 eter may be laid on the stage like an ordinary object-slide, or it may 

 be applied to the eye-piece, which, in the latter case, has to be specially 

 adapted for its reception. I shall deal exclusively with the simpler 

 stage-micrometer, which involves no alterations, and only costs 45. 6d. 

 If you know how to use this, with Beale's neutral-tinted glass, you can 

 accurately measure all the objects you see, and you can without diffi- 

 culty extend your accomplisiiments to the eve-piece micrometer and 

 more expensive cameras later on. The foot rule and compasses need 

 no special description. 



The method of using the accessories I have just referred to will be 

 most easily learned if 1 put a microscope in position for drawing. The 

 instrument is placed horizontally, the objective focussed, and the light 

 adjusted, the latter being an operation the best results of which can 

 only be acquired by practice, and in daylight ; the reflector is now 

 substituted for the cap, and so turned as to admit of the image formed 

 in it being projected, if I may so term it. on the table. If the light has 



