Maximum Length of tubes which this model can take is 400mm; smaller tubes can also be used. The tubes supplied have a bubble trap. Polaroid Sheets used for polarishing the incident light. Glass circular scale finely marked in angular degrees and also in ISS degrees. The o orange on ISS Scale is from -30 to +130 . The Scale is properly illuminated to facilitate reading. Permanent focus: Once the dividing line is focused for the eye. The focus is not disturbed by inserting tubes containing solutions, regardless of the nature of solution or of the length of the tube. Permanent good definition even with difficult liquids. Rotating half shadow Effect / Triple shadow effectProvision for attaching lamp to the instrument thus keeping alignment correct at all times.