McKay. — On Tclescojjir Piiulographii. 463 



8ired they should be. TJiis last defect I subsequently found 

 was due to a dissociation of the visual and actinic foci. But 

 I had so far succeeded in my original object, and, besides, had 

 made two discoveries which I rightly deemed to be of con- 

 siderable importance : first, that the focus on the screen of 

 the camera-box is, by use of the optical combination described, 

 controlled by the telescopic part, and that a very slight differ- 

 ence in the distance between the objective and the eyepiece 

 shifted the position of focus in the camera-box very consider- 

 ably ; second, that as the power of the eyepiece was varied 

 the size of the picture was affected, though in the case of 

 the higher powers there was necessitated a greater length of 

 the telescopic part. 



At this stage I made use of a IJin. objective lens, with an 

 eyepiece of such power that, with the screen at a distance of 

 Tin. from the back lens, a picture 6fin. by 4fin. was obtained. 

 And shortly I found that it was not necessary to use tlie photo- 

 graphic objective at all, and that the telescopic part of the 

 combination alone was suiiicient to produce a moderately 

 sharp picture ; but in using tlie latter I was much troubled by 

 the introduction of both chromatic and spherical aberration. 

 In the face of these difliculties I obtained a number of pic- 

 tures which under the circumstances might be considered as 

 very fair results. 



The instrument as it then was required a very small stop 

 to produce anything like sharpness in the photograph. I 

 have used a stop not more than -gVin. in diameter ; and, with 

 one of sVn., used to obtain results on a Wratten and Wain- 

 wright's " instantaneous plate " by giving an exposure of fron) 

 five to eight seconds. 



Th" difficulty now was that over and above the twofold 

 optical aberration of the instrument there was such poor illu- 

 mination of the screen that it scarcely sufficed for the cor- 

 rect focussing of the instrument ; and it might be, too, that 

 there was some slight displacement or the visual and actinic 

 rays. To obtain more light I had to make use of an objective 

 lens of larger aperture; and, this being of considerably longei- 

 focus, the consequences were that the two foci were disso- 

 ciated to such an extent that some compensation had to be 

 devised whereby to neutralize this effect. I overcame the 

 difficulty by constructing an eyepiece for use in the position 

 of the camera-screen, but which could be pushed forward into 

 the camera-box the distance required to obtain the correct 

 focus; and, this once determined, a stop-flange was so placed 

 that the same distance could be again found after this part of 

 the instrument had been withdrawn to make way for the dark 

 slide and prepared plate. 



The use of a 2-Jin. obj-etive lens of long focus with the 



