" THE AMERICAN 



MONTHLY 



MICROSCOPICAL JOURNAL 



Vol. V. 



Washington, D. C, May, 1884. 



No. 5. 



A Focussing Glass for Photo-Mi- 

 crography. 



Doubtless most of those who have 

 attempted photo-micrography have 

 experienced the same difficulty at the 

 outset as myself. I found that no 

 matter how finely my focussing glass 

 screen was ground, it would not al- 

 low the finer details of objects to be 

 seen. I had no focussing glass, my 

 half dozen eye-pieces were all posi- 

 tive, and in the absence of a Rams- 

 den, or negative, eye-piece, I de- 

 termined to make one of these do. I 

 took a narrow strip of thin board i J) x 

 2 X f inches, and in its centre cut a cir- 

 cular hole through which I could just 

 force one of my eye-pieces with con- 

 siderable pressure and a screwing mo- 

 tion (fig. 17). Now throwing back 

 my ground-glass screen and allowing 

 the projecting ends of the strip to rest 

 upon the edges of the camera box, I 

 had as clear and distinct an image as 

 in looking through a microscope. 



To adjust my glass to the position 

 occupied by the plate during expo- 



^::x. 



Fig. 17.— Photographic Focussing Glass. 



sure, I focussed with the regular 

 screen upon a printed text placed at 

 some distance from the camera, using 

 my ordinary view lens, being careful 

 to get the edges of the letters as sharp 

 as possible. Then throwing back the 

 screen, being careful not to change 

 the position of the bellows, I applied 

 my eye-piece with its carrier resting 

 against the edges of the box, and 

 screwed it in or out till the sharpest 



and clearest focus was obtained, mak- 

 ing a mark upon the eye-piece to 

 serve in case of accident, and the glass 

 was finished. Whenever I focus with 

 it I know that the image will fall ex- 

 actly upon the front surface of my 

 exposed plate. For microscopical 

 work I never think of using any 

 other means of focussing, as it gives a 

 clear, brilliant image, magnified and 

 exhibiting every minute detail, does 

 entirely away with the necessity of 

 the black cloth, and does not necessi- 

 tate the use of a plain glass screen as 

 does the ordinary focussing glass. My 

 first attempt with my glass was upon 

 P. angtilatmn^ which I proposed to 

 photograph by lamp-light with a \ 

 Tolles homogeneous immersion ob- 

 jective and a 2-inch ocular, the plate 

 being exposed less than 50 cm. from 

 the object, and giving an image only 

 37 mm. long. Upon the screen no 

 appearance whatever of the mark- 

 ings was visible, but with my glass I 

 was able to focus so that my nega- 

 tive showed the clear hemispherical 



Geo. O. Mitchell. 



[The plan above described is a 

 most excellent one, not only because 

 it enables the focussing to be done 

 very accurately, but because it also 

 enables certain corrections to be made 

 with ease. It will readily be seen 

 that when an objective is used which 

 is not well adapted to photographic 

 work, owing to the difference between 

 the focus for vision and that for ac- 

 tinic rays, the eye-piece can be so ad- 

 justed, by experiment, that when the 

 image is sharp as seen in the eye- 

 piece the actinic rays will be focussed 

 on the plate. — Ed.] 



