in which the meters are delivered is provided with a calibration device perforations on the side of the box 38 define a tab 39 whose removal reveals an opening 40 in which a slide 42 is slidably mounted by any appropriate means such as track defining internal rails 44 and 46 secured by spacers 48 and 50 to one wall 52 of the box 38 complementary edges 54 and 56 on the wall and the slide define the opening 40 so the size of the opening 40 varies with the position of the slide 42 length indicia 54 are printed on the box wall 52 adjacent the slide 42 and a pointing indicium 57 is printed on the slide 42 to point to them the size indicia in the illustrated embodiment are different colors say red blue green and yellow before a doctor uses a meter for the first time he fits the rings 20 and 22 on the bases of his fingers and places his fingers in the opening 40 with the slide 42 disposed in a position somewhat to the left in fig3 he opens his fingers until the meter reaches a predetermined reading sliding the slide to the right as he does so he then observes the size indicium 54 to which the pointing indicium 56 points when the meter reaches the predetermined reading and this is an indication of the relative size of his fingers best calibration is obtained when the doctor 39 s fingers are crooked in the manner in which they are crooked when he takes a dilation measurement when the doctor then uses the meter 10 to take an actual dilation measurement he notes the angle indicium.