230 PHOTO-MICROGRAPHY [CH VIII 



§371. Focusing Screen for Photo-Micrography. — Onecannot 

 expect a picture sharper than the image seen on the focusing screen. 

 Hence the greatest care must be taken in focusing. The general focus- 

 ing may be done with the unaided eye on the ground glass, but for the 

 final focusing a clear screen and a focusing glass must be used. (Figs. 

 172, 173). See § 347. With the clear focusing screen one cannot at 

 first see the image without using a focusing glass, but with a little ex- 

 perience the aerial image may be seen as with the microscope (§ 54). 



§ 372. Photo-micrographs of 20 to 50 Diameters. — For pic- 

 tures under 15 or 20 diameters it is better to use the camera for embryos 

 with the objective in the end of the camera, and the special microscope 

 stand for focusing (Fig. 175). 



For pictures at 25 to 50 diameters one may use the microscope with 

 a low objective, 25 to 35 mm. equivalent focus, and no ocular (Fig. 

 184). The object is placed on the stage of the microscope, and focused 

 as in ordinary observation. If a vertical microscope is used the light 

 from the petroleum lamp or other artificial light, is reflected upward by 

 the mirror. It may take some time to get the whole field lighted 

 evenly. Refer back to § 95 for directions. In some cases it may be 

 advisable to discard the condenser and use the mirror only. For some 

 purposes one will get a better light by placing the bull's eye or other 

 condenser between the lamp and the mirror to make the rays parallel 

 or even to make a sharp image of the lamp flame on the mirror. Re- 

 member also that in many cases it is necessary to have a color screen 

 between the source of light and the object (§ 356). 



For a horizontal camera it is frequently better to swing the mirror 

 entirely out of the way and allow the light to enter the condenser 

 directly or after traversing the bull's eye (Figs. 182, 186). If the ob- 

 ject is small an achromatic combination like a Steinheil magnifier or an 

 engraving glass is excellent (Fig. 188). When the light is satisfac- 

 torj r as seen through an ordinary ocular, remove the ocular. 



(A) Photographing without an Ocular. — After the removal of the 

 ocular put in the end of the tube a lining of black velvet to avoid re- 

 flections. Connect the microscope with the camera, making a light- 

 tight joint and focus the image on the focusing screen. One may make 

 a light-tight connection by the use of black velveteen or more con- 

 veniently by the Zeiss' double metal hood which slips over the end of 

 the tube of the microscope, and into which fits a metal cylinder on the 

 lower end of the camera (Figs. 184, 189, 183). In the last figure the 

 connection has been made. 



