Show and the eyen every one knows that in using a field glass it is necessary to adjust it to a proper focus suppose that you put one of the tubes at your focus and alic other tube at a focus that suited sonic one else and then you looked through both tubes you would have a more or less blurred vision and if you kept on looking the chances are that you would feel giddy and get a headache now the two eyes are supposed to have an equal natural focus and when by any chance that focus s unequal a headache results the remedy is a pair of classes or n single glass to make the eyes equal la power |